请教几个托福语法题

以下是引用luckygirlyaya在2005-8-14 13:56:00的发言：
1. Abraham Lincoln insisted that ___ not just on mere opinion but on moral purpose.

答案是 democracy be based ——insist that ... (should)....这是insist的一个固定用法

我选了 whenever democracy is based

2. 改错题

1）Observation of the Sun, Moon, and stars has enabled humans to determine both the seasons ans the time from day.

划线部分为：A. Observation B. humans C.and D.from

答案为D, 但我不明白为什么 ——意思是白天的时间(即长短)

2）The hardness of mineral often gives a clue to its identity.

划线为：A. The hardness B. of mineral C.gives D.to

答案为B,我蒙对了，但不明白为什么。 ——mineral应该是复数

多谢！！