The most important chemical catalyst on this planet is chlorophyll,______ carbon dioxide and water react to form carbohydrates.

A.whose presence

B.which is present

C.presenting

D.in the presence of which

为什么选D不选C呢?

______1810, water--powered textile manufacturing arrived in New Hampshire with the founding of a company in Manchester that manufactured cotton and wool.

A. Early

B.In the early

C.As early as

D.when early

答案是C,可是为什么不能选B呢?

1。因为空格后面是完整从句。

2。c表示动作持续，b指讲述一件过去的事情。