Diamond is the hardest of all materials.A. like we know B. since we knowC. we knowD. as

问题补充：

Diamond, , is the hardest of all materials.

A. like we know

B. since we know

C. we know

D. as we know

答案：

A. like we know

B. since we know

C. we know

(答案→)D. as we know

解析：定语从句题。先行词是整个句子时关系词只能用as，所以要选D。 正如我们知道的那样钻石是所有材料中最硬的。