请教几道语法题3Q

经一个朋友介绍偶来到这里

感觉真好~　大家都在互相讨论语法问题 不象有的论坛冷冷清清

请教几道语法题

1.For (each) enzyme reaction (there is) an optimum (temperature which) maximum (efficiency) is achieved.

answer : C  应该怎么改？

2.The (orbit) of a celestical (body) is (usually) in the shape (of ellipse).

answer : .D  应该怎么改？

3.Copper was the first (metallic) used by humans (and is) second only (to iron) in its utility through (the ages).

A错误很明显，但是B里面为什么能用is？前面是was…

...and... 的并列结构不是要前后一致的吗？

4.The first inhabitants of the territories (        ) Canada came across the Bering Strait and along the edge of the Arctic ice.

A. make up that now  B. make up now that

C. that make up now  D. that now make up

1　at which

2  ellipses

3  客观事实

4　谓语的一部分

2. of an ellipse