单选题 (That) troubles me is (that) I can't learn (all these) English idioms (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。