大全B第2题求助！

Modern critics are amused by early scholars’ categorizing Tacitus’s Germania as an ethnographic treatise.

(A) scholars’ categorizing Tacitus’s Germania as

(B) scholars’ categorizing Tacitus’s Germania as if

(C) scholars, categorizing of Tacitus’s Germania as

(D) scholars who categorize Tacitus’s Germania as

(E) scholars who categorize Tacitus’s Germania if

答案为什么是A啊？D为何不对呢？