欧美日韩黄网欧美日韩日B片|二区无码视频网站|欧美AAAA小视频|久久99爱视频播放|日本久久成人免费视频|性交黄色毛片特黄色性交毛片|91久久伊人日韩插穴|国产三级A片电影网站|亚州无码成人激情视频|国产又黄又粗又猛又爽的

Cloze test

There are many problems in the world, but the population problem may be the1 one of the world today. The world's population is growing 2 .Two thousand years ago, there were only 250 million people on the 3 Four hundred years ago, the number was 4 500 million. But at the beginning of the 5 century, the world's population was about 1,700 million. In 1970, this number was 3,600 million. In 1990, the number was five billion. A UN 6 said that the world's population had 7 six billion by the end of the 21th century. This is just ten 8 after it 9 five billion. People say that by the year 2010, 10 may be seven billion.