若M=2a2b，N=-4a2b，则下列式子正确的是 [ ]
A．M+N=6a2b
B．N+M=-ab
C．M+N=-2a2b
D．MN=2a2b
答案：C
连线题
小题1:将下列地区与对应的民居相连：
小题2:将下列省份与对应的旅游景点连线：
Not until he got to the factory _______that he had lost his papers.
A．he found
B．did he find
C．had he found
D．has he found
