阅读理解。 No air means death. Though we can't see it, there is air all around us, so air is everywhere and gives life to every living thing. Without it we can't live. Bad air makes people ill. We must have fresh air to keep us in good health. In the city there are many people and there are too many cars running on the roads the gas which the cars spread (传播) out is full of poison (有毒). Every day we breathe it in again and again. This makes us feel sick. Besides the cars, there are many factories too. From the chimneys (烟囱) of these factories, we usually get black and grey smoke. This kind of smoke, when mixed with air, is dangerous to our health because it contains much poisonous gas. So in order to keep ourselves healthy we should go out to the countryside to breathe more fresh air. 1. Air is very important because _______. [     ] A. We can't see it. B. It is everywhere. C. It gives life to every living thing. D. It does no harm to people's health. 2. We must have fresh air to _______. [     ] A. make our room healthy B. keep us healthy C. keep out house clean D. see every thing clearly 3. The gas which the cars spread out is _______ the black smoke that comes out of the factories. [     ] A. as grey as B. as much as C. as dense as D. as little as 4. In the city, the air is not so clean as that _______. [     ] A. in factories B. out of doors C. in the countryside D. indoors 5. We can't get more fresh air when we are in _______. [     ] A. hills B. countryside C. seaside D. big cities