# In a vast ocean region, phosphorus levels have doubled in the past few decades due to agricultural runoff pouring out...

#JW on October 4, 2019

D vs A

Can you please explain how D is ruled out and why A is the best choice?

Skylar on October 4, 2019

@Jwebb I'm happy to help! Let's take a look.

The first sentence of the given information states "In a vast ocean region, phosphorous levels have doubled in the past few decades due to agricultural runoff pouring out of a large river nearby." When we break this down, we can identify a cause and effect relationship indicated by the phrase "due to." The cause in this case is agricultural runoff, and the effect is the doubling of phosphorous levels. The next sentence states that "the phosphorous stimulates the growth of plankton near the ocean surface." Here we see another cause and effect relationship where the cause is phosphorous and the effect is the growth of plankton.

Therefore we know the following: Agricultural runoff causes a doubling of phosphorous levels, which in turn causes surface plankton growth.

Option (A) states that "the agricultural runoff pouring out of the river contributes to the growth of plankton near the ocean surface." We see from our breakdown that this is true, as the runoff causes the higher phosphorous levels which then cause plankton growth near the surface. Since agricultural runoff causes the cause of plankton growth, it is fair to say that it is a contributing factor. Therefore, (A) can be inferred and is correct.

Option (D) states "The quantity of agricultural runoff pouring out of the river has doubled in the past few decades." There is no information that directly supports this in the passage. We know that phosphorous levels have doubled, but this does not necessarily mean that the level of runoff has doubled as well. It is completely possible that the relationship between agricultural runoff and phosphorous levels are not exactly proportional. If this were the case, phosphorous levels could double in response to only a slight increase in runoff. Therefore, (D) is incorrect.

Does this make sense? Please let us know if you have any other questions.