請教2題pp文法....thanx

非常謝謝版主的連結，也下載了"文法大全"。不過仍有兩題不理解，是"文法大全"裡沒有說明到的。是否可以詳加解釋？謝謝！

1. By the mid-twentith century, United Stateds presidential staffs, ___ had numbered fewer than ten a century earlier, numbered in the hundreds.

a) as b) that they c) which d) and    選c  …不懂為什麼 staffs 是”人”(非”事物”)，為何可接which.

5.      It is said that no potential frontier has such fired the imagination or challenged the ingenuity of humankind as has space. 選C such …..對D選項不解，請問可以用什麼其他詞代替as嗎？（不懂為何as 後可接verb）