题目

Kate has been in hospital ______ she fell off a ladder and had her left arm broken.  A. from        B. because        C. since          D. after 答案：C家庭电路中“超负荷”是指电路中电流过大，出现上述情况时，电路中保险丝会自动熔断，是因为保险丝的熔点较______，其工作原理是利用电流的______效应．