Here are the answers based on the provided text:

1. **Which is a big problem in the city?**
- Traffic jams and pollution are big problems in the city.

2. **What causes traffic jams in the city?**
- Traffic jams occur when there are too many cars on the roads.

3. **How can cities reduce traffic congestion according to the passage?**
- Cities can encourage people to use public transportation, like buses or trains, instead of driving their own cars.

4. **What is suggested as a solution for pollution in cities?**
- One solution for pollution is to plant more trees and create more green spaces in the city.

5. **Can we make our city environment more sustainable and enjoyable?**
- Yes, by increasing the number of trees and using public transportation, we can make our city environment more sustainable and enjoyable.