this the only thing(A.that B.which C.what D.how) you most do well 选哪个

题目

this the only thing(A.that B.which C.what D.how) you most do well 选哪个
理由理由！！！！！为何不选B

答案

原句是：This is the only thing(A.that B.which C.what D.how) you most do well.
选A,定语从句中,先行词被only、very等词修饰时,关系代词不用which,而用that