题目内容：

There were _____ guests as chairs in the lecture hall.
A.so many B.twice so many
C.twice D.twice as many
凭语感我能选出来答案是 C ,但是我需要解释给别人听,A,B,D 为什么不对么?
对对，是选 我把答案看错了，那为什么是 D 该怎么解释其他几个是错的呢？