The reason their failure you know is they didn't get fully prepared for the experiment．
A. why, that
B. that, why
C. for, that
D. why, because
网友回答
A. why, that
B. that, why
(答案→)C. for, that
D. why, because
解析：
试题分析:重点在句子成分分析，is 为主句谓语。You know 为定从 that 引导表语从句。
for 表“对于”。句意：对于你知道的他们失败的原因是他们没有为实验做充足的准备。故选C