英语
初中 · 九年级
沪教版 · 九年级下册
填空题
★★★☆☆ 编号 #10763
用括号内所给单词的适当形式填空，使句子完整、通顺。
Air ___ (pollute) has become a serious problem in big cities.
Air ___ (pollute) has become a serious problem in big cities.