请教: 关于as...as后面的be动词

我最近发现在有些题里面as...as后面把be动词省掉了,但是有些题里面又不能省,不能搞懂为什么.谁能帮忙解释一下?多谢!下面我举两个Prep里面的题:

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

答案是B, was被省略掉了.

152 Recent research indicates that two popular arthritis drugs are possibly not as safe as they initially Believed.
(A) that two popular arthritis drugs are possibly not as safe as they initially believed
(B) that two popular arthritis drugs may not be as safe as they were initially believed to be
(C) two popular arthritis drugs to possibly not be as safe as it was initially believed
(D) the safety of two popular arthritis drugs may not be as great as it was initially believed
(E) the safety of two popular arthritis drugs not to be as great as they were initially believed to be

答案是B,were不能被省略.