单选题 ______ the explosion occurred was that the laboratory attendant had been careless.
A. That B. What C. Why D. How

【正确答案】 C
【答案解析】[解析] why引导的从句作主语，虽然that/how都可以引导从句作主语，但本题的表语从句表明了原因，因此why为正确答案。