[求助]大全249

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

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

答案D，请问E错在哪？