6.     The guiding principles of the tax plan released by the Treasury Department could have even a greater significance for the economy than the particulars of the plan.

(A) even a greater significance for the economy than

(B) a significance that is even greater for the economy than

(C) even greater significance for the economy than have

(D) even greater significance for the economy than do（D）

(E) a significance even greater for the economy than have

我选C, 本题答案是D.

但D中的do到底和什么词平行对称呢?

整句主干是the principles could have greater significance than the particulars （could省略）have.

do代替have，与前面平行

谢谢!

我懂了,只有完成时中助动词用have. 比如 I have studied a lot, as she has.

因为这里的could have greater significance中have不是完成时而是表达"有",助动

词应该用do.

整句主干是the principles could have greater significance than the

particulars （could省略）do.