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
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A. why.., that
B. that.., why
(答案→)C. for.., that
D. why.., because
解析：此题关键是their failure为名词词组，A项why后须接句子。