一道语法题的困惑

14. Nearly all trees contain a mix of polymers that can burn like petroleum --- properly extracted.

(A) after

(B) if

(C) when it

(D) is

答案：B

分析：当if引导的条件状语从句的主语和主句的相同时，可以省略主语和助动词。类似的还有though、although和when。（B）相当于if they are。

（A）可以引导从句，但缺少从句主语和助动词；

（C）缺少表被动的助动词；

（D）不能引导从句。

参考译文：几乎所有的树都含有一种混和的聚合体，这种聚合体经过合适的提炼后可以像汽油一样燃烧。

我的困惑是状语从句的省略要符合两点:主句主语与从句主语一致;从句的谓语必须是be动词,可是这道题的从句谓语应该是extracted,难道extracted在这里是形容词?

Extracted是extract的过去分词呀。

还有一题:

Maya Angelou’s widely acclaimed autobiography, I know why the Caged Bird sings, is a

moving and ______ of her childhood in segregated Arkansas.

(A) an account that is often humorous

(B) often humorous as an account

(C) often humorous, the account

(D) often humorous account

答案是D

is a moving and often humorous 后面是不是省略了什么?moving和humorous都是adj啊