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A - Correct. Between sixty and fifty mill years, the whales were dependent on fresh water, so even if they had ventured to salty waters they would have had to swim back to fresh water if they were to survive. It is REASONABLE to assume that the whales wouldn't travel FAR from their fresh water habitat for obvious reasons. So fossils of whales from this period would only be found close to Asian Fresh water locations and not in far off places.
B - As a rule of thumb, always be suspicious of choices that use strong words such as NONE, ALWYAS, NEVER, ALL etc. These are USUALLY incorrect.
Now while it would be plausible to assume that MORE whales fossils from later than about 50 mill years would belong to SALTWATER whales yet we have no EVIDENCE to believe that ALL FRESHWATER Fish would have disappeared from earth between 60 to 50 Million years. So B is out.
C - Again there's no evidence in the passage that says that such a thing is possible; remember that the whale always had a choice of moving back and forth between fresh and saltwater - so it is not unlikely for both fresh and salt water whales to have cohabited the same part of the ocean at one time or the other. Again the Operative word is "WILL NOT BE FOUND" which is very extreme. If the choice had said - it was unlikely for fossils of fresh water whales to exist with fossils of Saltwater whales - it may have been true.
D - again ONLY in their possession of specialized kidneys - while the fact that they differed with respect to kidneys has been established in the passage, a categorical stmt like that KIDNEYS were the only point of difference cannot be supported from the passage
E - There's no evidence in the passge to support this kind of an invasion.
(A) is correct here. I also believe the argument expects us to assume that "continental drift" wouldn`t slowly move the whale fossils great distances over millions of years. Sure whales can swim far, but how far could they go without stopping for a drink??