大全356 twice as much of...as中间那个of啥用途

  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

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

(C) twice as much of its 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

答案是C,没有意见。但是C中的第一个of有什么用途呢？请教。

不愚蠢哦。我也对as much of its national output of goods as 这里面的东西不理解。