20.We cannot figure out ____quite a number of insects, birds, and animals are dying out.

A.     that                B. as                      C. why          D. when

【解析】C：figure out=understand by thinking. 理解。从句意看我不明白大量物种正在消亡，缺少状语，排除A、B项。从are dying out的时态排除D项，应该是我不明白大量物种为什么正在消亡。