The reason he felt frustrated was he lost his MP4.
A. why/that
B. that/why
C. why/because
D. that / because
网友回答
(答案→)A. why/that
B. that/why
C. why/because
D. that / because
解析：句意：他郁闷的原因是他丢了他的MP4。The reason why…is/was that…意为：…….的原因是……。why he felt frustrated是定语从句修饰the reason，that he lost his MP4是表语从句。故选A。
考点: 考查名词性从句的用法。