题目

If he_____ to the teachers attentively ,he _________ the answer to the problem now. A had listened; would have known B listened; would know C listened; would have known D had listened; would know   答案：D导致儿童发育停滞智力低下病症，主要是因为缺乏的元素是A.碘B.铁C.锰D.锌