我国宪法规定，中华人民共和国的根本制度是[ ]
A．人民民主专政制度
B．人民代表大会制度
C．社会主义公有制
D．社会主义制度
答案：D
I''m surprised that he ____ to help me when I needed his help .
A．should refuse
B．would have refused
C．should have refused
D．may have refused
多级离心泵的效率比单级泵要低
