[求助]GWD13的一道语法

Q21:

Despite the growing number of people who purchase plane tickets online, airline executives are convinced that, just as one-third of bank customers still prefer human tellers to automatic teller machines, many travelers will still use travel agents.

还有一题

Q29:

For protection from the summer sun, the Mojave lived in open-sided, flat-topped dwellings known as shades, each a roof of poles and arrowweed supported by posts set in a rectangle.

1. each a roof of poles and arrowweed
2. each a roof of poles and arrowweed that are being
3. with each being a roof of poles and arrowweed
4. with roofs of poles and arrowweed to be
5. with roofs of poles and arrowweed that are

答案是A,我选拉E,为什么不对啊

自己顶,请大家帮忙解释~

Q21:

我选这道题时根据convince + that clause 判断答案，就把b,c, d, 给排除了；然后根据despite 通常只跟简单名词，而不会接很长的句子或一连串的动名次或分词或其他修饰成分的原则，把e 排除。

Q. 29

E.    如果你要表达shades特性的话，with 短语前的逗号应该省略的。

   另外，that 的修饰对象不明。