阅读下列短文，从每题所给的A、B、C、D四个选项中选出最佳选项。
A
The United States has more tornadoes (龙卷风) than any other country in the world. In a normal year there are 800 or 1,000 tornadoes. Most happen in the middle part of the country. Tornadoes form when warm and cool air meet. In the Midwest，the warm air from the Gulf of Mexico often meets the cold air from Canada.
The usual tornado season is March through May. Tornadoes form most often in the afternoon and early evening. There is often no warning of a tornado. People who live in the Midwest know the signs (征兆) of tornado activity. The sky becomes dark，often a greenish color. Dark clouds appear in the sky and there is often a large hail (冰雹). Suddenly，there is a loud sound，like a train or a jet plane. Sometimes，two，three，five，ten or more tornadoes can form over a large area.
This type of tornado activity hit Oklahoma and Kansas in May 1999．The day was stormy，with heavy thunderstorms in the afternoon. As the storms continued，tornadoes began to form. Dozens of tornadoes hit towns and neighborhoods in Oklahoma，then in Kansas. Some stayed on the ground for several hours，destroying everything they touched. The tornadoes killed 43 people and injured 600 others. They destroyed thousands of homes and businesses. In some areas，not one home stood. In other areas，the tornadoes destroyed every home on the left side of the street，but didn’t touch any homes on the right side. The tornadoes lifted people and cars into the air and then threw them back down to earth. So it is important to know where to hide and how to protect yourself in the face of a tornado.
文章大意：本文是一篇说明文。文章主要就发生在美国的龙卷风进行了简单的介绍。
21．What can we learn about tornadoes in America？ __A__
A．They may appear in groups.
B．They usually happen in the summer.
C．There are more tornadoes in the Eastern States.
D．There are more than 1,000 tornadoes every year.
解析：细节理解题。由第二段中的最后一句“Sometimes, two, three, five, ten or more tornadoes can form over a large area. ”可知，这些龙卷风有可能会成群出现。故选A项。
22．The tornadoes that hit Oklahoma and Kansas in May 1999 __D__ .
A．took place at the same time B．were the biggest in history
C．came and went quickly D．brought death and destruction
解析：细节理解题。由末段中的“destroying everything…600 others”可知，D项正确。
23．What will the author most probably talk about next？ __B__
A．Tornado dangers and warning signs.
B．How to stay safe during a tornado.
C．Different kinds of tornadoes.
D．How a tornado forms.
解析：推理判断题。由末段中最后的“So it is important to know where to hide and how to protect yourself in the face of a tornado. ”可知，作者接下来很可能谈论在龙卷风来临时如何保护自己。故选B项。
