169. 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
some time also can be omitted in"not only ...but also", and the expression is correct.
in 169,A is wrong because when "prove" is a transitive verb it means:to show(oneself) to be worthy or capable,It should followed by an object.otherwise,it would be "prove to be"in this status.
C is wrong because it should be a "that" follow believe. believe that believe sth. believe to be...