We cannot figure out quite a number of insects, birds, and animals are dying out.
A. that
B. as
C. why
D. when
网友回答
A. that
B. as
(答案→)C. why
D. when
解析：：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。
【标签】名词性从句连接词