I think we should choose B. First, take an A 2 from the front and make it (a-5c) 2 from the back, then there is an A 2 and two items from the front, and combine these two items into 1/B(A-B). Then, compare the inequality of B (A-B) and regard (A-B) as a whole. = (b+a-b) 2/4, that is, 4/a 2, so there is a 2+a 2/4 > = 4. Since (a-5c) 2 can be equal to 0, the answer should be B.