SEC 1-23

23.   In 1980 the United States exported twice as much of its national output of goods as they had in 1970.   

(A) twice as much of its national output of goods as they had done

(B) double the amount of their national output of goods as they did

(C) twice as much of its(United States’) national output of goods as it did

(D) double the amount of its national output of goods as it has（C）

(E) twice as much of their national output of goods as they had

Choice A is incorrect because the plural pronoun they does not agree with its singular noun referent, the United States, and because had cannot substitute for exported. Choice C is best: the form of the comparison is idiomatic, the pronouns agree with the noun referent, and did—the simple past tense of do—can substitute for exported.

白勇的书里说，主句谓语动词使用助动词（have,had,will）或情态动词,比较从句可以省略verb，仅仅保留助动词。

这里为什么？？？？？？？？？