# The populations of certain species of amphibians have declined dramatically in recent years, an effect many scientist...

Ryan-Mahabir on October 21, 2019

Why is A correct? Why is C incorrect?

Thanks

Replies

BenMingov on November 13, 2019

Hi Ryan, thanks for the question!

For necessary assumption questions, we negate the answers we are considering and test whether they will damage the conclusion.

When we negate answer choice A, it takes on the form of the following: The amphibian species whose population declines have been attributed by many scientists to industrial pollution ARE known to be among those species whose population do not vary greatly as a result of natural variations in the weather"

This is a very long way of saying that their populations do not vary greatly due to natural temperature fluctuations.

If this is the case, then it isn't impossible to determine whether this population decline was due to industrial pollution, because we no longer have to consider the impact of temperature fluctuations.

As for answer choice C, why would it be necessary that it cannot be a combination of the two reasons that led to the decline in the amphibian population. This does not have to be true.

Hope this helps!

tomgbean on December 6, 2019

Hi Ben,

Wouldn't the correct negation of "The amphibian species whose population declines have been attributed by many scientists to industrial pollution are not known to be among those species whose populations do not vary greatly as a result of natural variations in the weather," be "The amphibian speciies whose populations declines have been attributed by many scientists to industrial pollution ARE known to be among those species whose populations DO vary greatly as a result of natural variations in the weather?" The negation is suppose to destroy the argument.

shunhe on December 23, 2019

Hi @tomgbean,

The double negation you used actually does not negate the sentence completely; BenMingov has the correct negation. Think about it this way (using a completely made-up example):

The fries at McDonald's are known to be among the fries which have the highest sodium content.

The correct negation of this only negates the "are," which is the main verb of the sentence. In other words, the opposite of this sentence is "The fries at McDonald's are not known to be among the fries which have the highest sodium content."

Your negation effectively makes the sentence this: The fries are McDonald's ARE NOT known to be among the fries which DO NOT have the highest sodium content, which has a different meaning. Hope this helps!