Why the explosion occurred was the laboratory attendant had been careless.
A. for
B. because
C. since
D. that
网友回答
A. for
B. because
C. since
(答案→)D. that
解析：考查表语从句：句意：爆炸发生的原因可能是实验室管理员太粗心了。Why引导的是主语从句，was后面是表语从句，因为成分和意思都很完整，用that引导表语从句，for不能引导表语从句，because因为，since自从，选D。