题目内容：

pollution is a serious problem in big cities throughout the world 同义句转换就变throughout这点

优质解答

all over /all around/round
Pollution is a serious problem in big cities all over the world .
Pollution is a serious problem in big cities all round the world .
Pollution is a serious problem in big cities round the world .
句意:污染是全世界大城市的严重问题.