Tom, clean up the broken glass _________ someone walks on it.

A.since       B. when       C. before     D. as

The clean-up operation at the bullet train crash site in east China's Zhejiang province has been completed. Meantime, trains have resumed services on the railway ______ the accident occurred.

Clean air is made up of only nitrogen, oxygen, and water. As we have industrialized, our cities have grown and traffic has increased. With these development there has come an increase in the number of by-products released into the air, such as particles（细粒）from burning fuels and poisonous gases from chemical reactions. These gases become pollutants.

Air pollution has a bad effect on the earth and our environment. Air pollution can result in acid rain, and it can also lead to the Greenhouse Effect—a condition in which pollution stops radiation from traveling out of the Earth’s atmosphere. This upset to the natural process means that temperatures on Earth may not be maintained within natural ranges. Due to the Greenhouse Effect, temperatures on Earth become warmer, which is known as global warming. What’s worse, air pollution may produce holes in the ozone layer（臭氧层）— this layer of ozone naturally prevents ultra-violet rays （紫外线） from the sun reaching Earth. Holes make it possible for the rays to reach us.

Don’t let all the bad news about air pollution get you down because there are many ways to help fight the air pollution. Every one of us can employ a variety of air pollution solutions. A smoker can give up smoking. While shopping, you can choose organic and natural cleaning products. Driving less and carpool more can also contribute to less pollution. Next time, keep your air-conditioning low. Of course you can plant more trees.

Air pollution is a major problem for our planet. However, as long as we realize how serious the air pollution is and take actions now, everyone can make a difference!

                     Title:71.         

The clean-up operation at the bullet train crash site in east China’s Zhejiang province has been completed. Meantime, trains have resumed services on the railway ______ the accident occurred

1. A.
   which
2. B.
   that
3. C.
   where
4. D.
   the place where

Tom, clean up the broken glass ______ someone walks on it.

1. A.
   since
2. B.
   when
3. C.
   before
4. D.
   as