The conclusion of this argument is the last sentence, i.e. "Thus, photovoltaic power plants offer a less expensive approach to meeting demand for electricity than do traditional power plants." How do we know this? We are told that the cost of producing electric power at photovoltaic power plants is one-tenth of what it was 20 years ago, whereas the cost for traditional plants has increased.
Notice there is a huge gap in this argument. We never compare the cost of producing power at photovoltaic power with the cost of producing power at traditional power plants. Instead the author is comparing the cost at photovoltaic power plants today versus 20 years ago. Then the author compares the cost at traditional power plants today versus 20 years ago. For this argument to make sense, we need to somehow relate the cost at photovoltaic power plants with the cost at traditional power plants.
This is the Strengthen with Sufficient Premise question so we are looking for the answer choice that fills this gap and 100% guarantees the conclusion.
Let's take a look at the difference between (A) and (D):
Answer choice (A) states that the cost of producing electrical power at traditional power plants have increased over the past 20 years. This cannot guarantee the conclusion because it is nothing new. We have already been given this information in the stimulus, i.e. "whereas the cost for traditional power plants has increased."
Answer choice (D), on the other hand, guarantees the conclusion by relating the cost at photovoltaic plants to the cost at traditional plants. Some simple numerical examples will help clarify this.
Assume that 20 years ago, the cost at photovoltaic plants was $100. Today it is one-tenth of that, or $10. Now if $100 was less than 10 times the cost of producing power at traditional plants (e.g., assume cost at traditional was $11 to ensure that $100 was less than 10 times the cost of producing power at traditional plants, i.e. $100 < $110), then today, it would have to be true that "photovoltaic power plants offer a less expensive approach to meeting demand for electricity than do traditional power plants." With our numbers, today a photovoltaic power plant would cost $10 whereas a traditional power plant would be at least $11 (at least because the stimulus also told us that "the corresponding cost for traditional plants...has increased").
Hope this helps! Please let us know if you have any other questions.
Ro13July 24, 2018
Okay so I know that D is the answer, but why is C wrong? To me, it's making sense to say that by guaranteeing that these recent technologies that are used to lower the cost of photovoltaic cannot be used to lower traditional power plants cost.
ChristopherJuly 27, 2018
@Ro13, (C) doesn't work because even if the new technology could be applied to traditional power plants, the question specifically says that the cost of running traditional power plants had gone up in the same time that photovoltaic plants had become cheaper. Therefore, whether the new technology could be applied to traditional power plants is essentially irrelevant.
