[讨论]not only but also一道题，关于also省略

1.     
Ignoring the admonitions of his
staff, the chief financial officer accepted the advice of the consulting
company because he believed that the standardized accounting procedures
would prove not only inexpensive but reliable indicators of economic
performance.

(A) he believed
that the standardized accounting procedures would prove not only inexpensive
but

(B) the
standardized accounting procedures will prove both inexpensive and also

(C) he believed
the standardized accounting procedures would prove themselves to be both
inexpensive and

(D) he believed
that the standardized accounting procedures would prove to be both inexpensive
and（D）

(E) standardized
accounting procedures will prove his belief that they are both inexpensive and

记得在语法专区中以前有讨论说在not only……but also 结构中also可以省略

本题答案是D,可是我怎么看A也正确啊，如果also省略的定理存在，难道a有其他错误？请各位讨论下

今晚刚做到这道题。。。

A应该是prove to be省略了成prove不好吧。