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.