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?
(A) Fossils of whale species dating from between sixty million and fifty million years ago will not be found on continents that were at the time separated from ancient Asia by wide expanses of ocean.
(B) Among whale fossils that date from later than about fifty million years ago, none are fossils of whale species that drank only fresh water.
(C) Fossils of whale species that drank fresh water will not be found in close proximity to fossils of whale species that drank salt water.
(D) The earliest whales that drank salt water differed from fresh-water-drinking whales only in their possession of specialized kidneys.
(E) Between sixty million and fifty million years ago, the freshwater lakes and rivers in which whales originated were gradually invaded by salt water.
Mapping:
Whales originated in freshwater lakes/rivers of Asia 60M yrs ago
+
Whales in the group of 60M-50M years did not have specialized kidneys to deal with ocean-saltwater --> Fossil of this group shows that they OCCASIONALLY went to salt-waters --> BUT eventually had to return to freshwater to drink
Conclusion:
This 60M-50M group must have had to return frequently to freshwater rivers to drink
(as per the fossil evidence).
We need a answer choice that strengthens this claim.
(A) Fossils of whale species dating from between sixty million and fifty million years ago will not be found on continents that were at the time separated from ancient Asia by wide expanses of ocean.
Close contender. This states that none of the fossils were found near continents which were surrounded with salty water, implying that all had to return to the freshwater lakes and rivers near Asia to drink
(B) Among whale fossils that date from later than about fifty million years ago, none are fossils of whale species that drank only fresh water.
Details about 50M+ group is irrelevant. We are concerned with 60M-50M only. Out of scope
(C) Fossils of whale species that drank fresh water will not be found in close proximity to fossils of whale species that drank salt water.
Tricky, and a trap answer 
. This actually provides details about two types of fossil but we are concerned with only one to answer the target question. We have to prove that whales returned to freshwater. This answer choice just mentions that the the fossils from salt water were not found near the freshwater fossils, in a sense restating what is given (some swam to the oceans traveling a certain distance)
(D) The earliest whales that drank salt water differed from fresh-water-drinking whales only in their possession of specialized kidneys.
Other differences are invalid, and in a sense restating whats already given
(E) Between sixty million and fifty million years ago, the freshwater lakes and rivers in which whales originated were gradually invaded by salt water.
A possible reason why the later species developed specialized kidneys, we need information about 60M-50M group
Between A and C, A provides additional information so the Answer is A
