单选题 That troubles me is that I can't learn all these English idioms by heart.
A. That
B. that
C. all these
D. by heart

【正确答案】 A
【答案解析】that引导主语从句时，不在从句中充当成分。而本句中需要在主语从句中充当主语的关联词来引导这个主语从句，因此that错。可以改为what。