1.This app allows you to review lessons by playing games.               2.We were cleaning up the beach when it started to rain / raining.      3.Ecotourism provides tourists with opportunities to explore nature.

2 We were cleaning up the beach when it started to rain

3 Ecotourism provides tourists with opportunities to explore nature

Air pollution is one of the most serious environmental problems in Viet Nam. There are many bad effects that air pollution brings to our lives, so it is important that we take steps to reduce it. 

The main issue with air pollution is its impact on public health. Pollutants in the air can lead to breathing diseases and heart problems. Additionally, air pollution damages the environment, harms wildlife, and contributes to climate change. 

To solve this problem, we should encourage the use of public transportation. This can significantly reduce the number of private vehicles on the road, leading to lower emissions. In addition, the government should ban stricter laws on industrial emissions and to promote the use of clean energy sources. 

In conclusion, even though air pollution is a big problem, we can make things better by using buses more and making factories cleaner.