Prep破解版 1-142

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
(B) as the average American to be self-employed
(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

A和B的区别在哪里？

Google了一下。是因为主语不parallel？
A. "Americans of Middle Eastern descent" vs. "national average"
B. "Americans of Middle Eastern descent" vs. "the average American"

A中的"national average"该怎么解释呢？