网友回答：
it is a serious threat to us because.it means the poisoning of the air， rivers and lands，there are many factories that bring pollution to us. we can not have fresh air in the morning. this is mainly because of the poisonous smoke coming from factories，相反
温馨提醒：随时光变迁，文中所陈述观点；准确性、难免有所变动，因此文中观点仅供参考。
