Who ___________ this man? [ ]
A. am
B. is
C. are
答案:B
实验室制取二氧化碳,只能用______法收集,不能用______法收集,原因是______.
泵按传送介质分类,可分为()
A.清水泵
B.立式泵
C.污水泵
D.油泵
E.耐腐蚀泵