第四部分 完形填空（2010.4）1The population problem may be the _1_ one of the world today. The worlds population is growing _2_ . Two thousand years ago, there were only 250 million people _3_ the earth. Four hundred years ago, the number was _4_ 500 million. But at the beginning of the _5_ century, the worlds population was about 1700 million. In 1970, this number was 3600 million. In 1990, the number was five billion. A _6_ report says that the world population will _7_ six bil
