空气中含有（）以及各种烟尘等，都会促进金属的锈蚀。
A．N2
B．H2S
C．SO2
D．HCI
参考答案：B, C, D
Don't go out to play. You _____ finish your homework first. [ ]
A. must
B. have
C. had
D. had to
明晰产权是政府治理垄断的微观经济政策之一。()
