zaarathelab wrote:Whales originated in the freshwater lakes and rivers of ancient Asia about sixty
million years ago. Not until about ten million years later did species of whales develop specialized kidneys enabling them to drink salt water. Although fossil
evidence shows that some early whale species that lacked such kidneys
sometimes swam in the Earth's saltwater oceans, these species must have had to
return frequently to freshwater rivers to drink. Which of the following is most
strongly supported by the information given?

zaarathelab wrote:Mitch, although the info in the paragraph states that these species must have had to return frequently to freshwater to drink, what if they couldn't return and died in the saltwater (since they can't drink saltwater.) Their bodies could have been washed away to the salt water side of the shore. fossilizing over millions of years.

Am I thinking too much?

GMATGuruNY wrote:

zaarathelab wrote:Whales originated in the freshwater lakes and rivers of ancient Asia about sixty
million years ago. Not until about ten million years later did species of whales develop specialized kidneys enabling them to drink salt water. Although fossil
evidence shows that some early whale species that lacked such kidneys
sometimes swam in the Earth's saltwater oceans, these species must have had to
return frequently to freshwater rivers to drink. Which of the following is most
strongly supported by the information given?

On the GMAT, the correct inference is the answer choice that MUST BE TRUE.
One strategy is to apply the NEGATION TEST. The negation of the correct answer choice will directly contradict information in the passage.

Answer choice A, negated: Fossils of whale species dating from between sixty million and fifty million years ago WILL BE FOUND on continents that were at the time SEPARATED FROM ASIA BY WIDE EXPANSES OF OCEAN.

The negation of A contradicts the following quote from the passage: These species must have had to return frequently to freshwater rivers to drink.

Since the early whales originated in Asia and needed access to freshwater rivers, they could not have migrated across wide expanses of ocean to other continents. Thus, the negation of A -- that fossils of these early whales will be found on faraway continents -- is impossible.
Since the negation of A contradicts the passage, A is the correct inference: the answer choice that MUST BE TRUE.

The correct answer is A.