多项选择题The problems need to be avoided in choosing a research topic（ ）.A.The topic is not interesting.B.The topic is too broad.C.The topic is too narrow.D.The topic is too hard.
单项选择题冬运期间凡因天气严寒，必须减吨时，铁路局管内的可根据具体情况按牵引定数减()
