The passage implies that the positive-displacement pump differs from the centrifugal pump in that the positive-displa...

Victoria May 14, 2019

Hello @Samantha-Alexis,

You are correct in identifying why answer choice C is wrong.

The issue with answer choice A is subtle, but essential. Answer choice A says that the positive-displacement pump "is more promising, but it also is more expensive and demands more maintenance." The key word to focus on here is "more." The passage does say that the positive-displacement pump is promising and expensive, but it does not directly compare it to the centrifugal pump in this sense. We know that the centrifugal pump requires little maintenance, but this is not directly compared to the positive-displacement pump. Therefore, we cannot say that the positive-displacement pump is more promising, more expensive, or more maintenance-intensive than the centrifugal pump as we do not know the promise or cost of the centrifugal pump nor the required maintenance for the positive-displacement pump.

E is the correct answer because the passage indicates that the centrifugal pump separates the oil from the gas at the beginning of the process, meaning that the crude would have less contact with the pump. In comparison, the passage explicitly states that the positive-displacement pump's design "brings crude into contact with most parts of the pump."

Hope this was helpful! Please let us know if you have any further questions.

Meredith September 7, 2019

How does separating oil from gas mean crude would have less contact with the pump? Where is that supported?

Victoria October 2, 2020

Hi @Meredith,

Happy to help!

It is implied that the centrifugal pump does not involve crude coming into close contact with the pump because this is presented as a downside of the positive-displacement pump but is not mentioned when discussing the centrifugal pump.

We can infer from the passage's comparison of the two pumps that the centrifugal pump is superior to the positive-displacement pump because it does not have to be made from expensive corrosive-resistant material. Why? Because the pump is not as exposed to crude.;

Therefore, while this answer choice is not explicitly supported by the passage, it is correct as it is used as a point of comparison between the two pumps.

Hope this helps! Please let us know if you have any further questions.

AneeshU June 8, 2022

@victoria

So according to my understanding of your explanation, A is wrong because even though a positive-displacement pump is promising, expensive, and high maintenance, there is nothing to show that it is comparatively more promising, expensive and high maintenance than the centrifugal pump.

However, the justification you've provided for E is that the passage doesn't mention that the centrifugal pump isn't as exposed to crude.

Therefore, both options rely on information being provided about one type of pump and not the other, while no comparative statement is made. The justification that you've relied on for why E is the correct answer, i.e. "It is implied that the centrifugal pump does not involve crude coming into close contact with the pump because this is presented as a downside of the positive-displacement pump but is not mentioned when discussing the centrifugal pump" is also applicable to A - We can infer that the positive displacement pump is more promising (line 31), and expensive (line 38) simply because they are not mentioned when discussing the centrifugal pump. Similarly, we can infer that the positive displacement pump is higher maintenance simply because the passage says that the centrifugal pump is low maintenance (line 43) and since the passage is silent as to the cost of maintenance of the positive displacement pump.

Could you please help me understand how the connection between the two is not the same?

Emil-Kunkin June 13, 2022

Hi AneeshU,

You ask a good question, and I think the support for the idea that the Centrifugal pump does not have many parts exposed to crude is a bit tough to find.

We are told that (regarding the PD pump)'s regular contact with crude requires it be made with expensive corrosion-resistant material. The author then contrasts it with the C pump. While it is not explicit, the fact that the second paragraph is structured as "PD has pros, but also the con of oil coming into contact. The alternative is C, which has these pros and these cons" does imply that they have different cons- so I think one could reasonably infer that the C pump does not necessitate expensive corrosion resistant material.

I agree that we cannot prove E is completely correct, but the question stem only asks what the passage implies- not what it explicitly says. A is never implied because we have no idea about the relative costs/promise of the pumps.

AneeshU June 14, 2022

Thanks Emil. I agree that the justification for E is as solid as the justification for A.

I'm looking for a differentiating factor between the two which will put one over the other because I'm fairly confident that both are implied in the passage(see below). At my current level of understanding I only have a 50/50 chance of getting this question right because both look like good answers to me.

Regarding the relative costs/promise,

"brings crude into close contact with most parts of the pump, and thus requires that it be made of expensive corrosion-resistant material," substantiates the 'cost' part of the option (since most parts of the pump need to be made with an expensive material)

and

"Of the two pumps being redesigned, the positive-displacement pump is promising because it is immune to sudden shifts in the proportion..." substantiates the 'promising' part of the answer choice.

Emil-Kunkin June 23, 2022

Hi AneeshU,

You are right that the PD pump is promising, and that the PD pump is made of materials that are costly, but we are not able to compare these things to each other. We have no idea how promising the C pump is. The PD pump is promising, but is it more promising that the C pump? We are never told this.

We also know that the PD pump is made of costly materials, but we cannot say that it is more costly than the C pump. Maybe the C pump is made of even more expensive materials. Maybe the C pump has much higher operating costs than the PD pump. A is incorrect because it makes a relative claim about two things when we only know about one of the things.