最近，南宁市人民政府发出通告：6月2日禁止摩托车在市区内行驶一天．从中可测定摩托车对空气污染的情况．机动车影响城市空气质量的气体污染物主要是（　　）

A．CO₂、O₂、N₂

B．SO₂、NO₂、CO

C．CH₄、CO₂、SO₂

D．HCl、CO₂、N₂

I          use a clock to wake me up because at six o'clock each morning the train comes by my house.

A．couldn't

B．mustn't

C．shouldn't

D．needn't