Since the explanations offered are ------- to the exposition, it would be unfair to treat them as ------- parts of the studies under consideration.
　　A.tangential . . subsidiary
　　B.irrelevant . . superfluous
　　C.referable . . correspondent
　　D.incidental . . essential
　　E.crucial . . immutable
　　Here's Why:
　　The words filling the blanks are opposites, since if the “explanations” are one thing, it is “unfair to treat them” as the opposite. They are “incidental” and cannot be treated as the opposite of incidental, namely, “essential.”