THE CORRECT ANSWER IS D. THE QUESTION ASKS WHY FILDES AND WHITAKER...

28. The correct answer is D.

The question asks why Fildes and Whitaker conducted the

experiment with liquid culture. According to the passage, "because

of the possibility that the lack of uniformity of the agar cultures had

in fact trained mutant strains, they conducted similar experiments

with liquid cultures..." So Fildes and Whitaker used liquid culture

because agar culture was not necessarily uniform. This is consistent

with Choice D.