请教PREP1 142

142.     (32496-!-item-!-188;#058&006253)

According to a 1996 study published in the Journal of Human Resources, Americans of Middle Eastern descent were twice as likely as was the national average to be self-employed.

(A) as was the national average to be self-employed 跟americans不对等，不能省略掉american

(B) as the average American to be self-employed 这里的was是作表语，后面没有宾语，主语作比较是可以省略掉be动词，因为不会有相同的名词来引起歧义。

(C) as the national average that they would be self-employed

(D) to be self-employed than was the average American

(E) that they would be self-employed than was the national average

这道题目的答案是B，我想问一下这道题目的比较对象不是Americans of Middle Eastern和the average American吗？那为什么可以省略was啊？谢谢大家。