Đề bài; Hoàn chỉnh các câu sau
1. If the air / in / city / be / polluted / , / it / can / cause / people's / respiratory / problems
2. If / smog / be / frequently / formed / in / city / , / it / cause / difficulty / in / breathing / headache / even / lung caner
3. If / carbon monoxide / concentrate / in / great/ amounts / , / it / be / harmful
4. the climate / change / if / more trees / be / cut / down / for / hardwood
5. If / we / not / control / pollution / soonn / , / it / be / too / late
6. If / we / not / protect/ frontier / , / we / suffer / many / from / natural / disasters
