单选题 When the engine would not start, the mechanic inspected all the parts to find what was at ______.

【正确答案】 C
【答案解析】[解析] at fault是固定搭配，意为“出故障，出差错”。A项wrong意为“错误的”，是形容词，不与at搭配；B项trouble与in搭配；D项difficulty也与in搭配，不与at搭配。