We wonder quite a number of living things are dying out.A. thatB. asC. whyD. when

问题补充：

We wonder quite a number of living things are dying out.

A. that

B. as

C. why

D. when

答案：

A. that

B. as

(答案→)C. why

D. when

解析：考查宾语从句。句意：我想知道为什么有如此多的生物灭绝。That在宾语从句中不能充当任何成分，之前连接作用；as不能引导宾语从句。When的意思不搭配。故C正确。