下列句子中，每句都有一个错误，请找出错误之处并改正。
1. The price in this city is very expensive. ______
2. We must do all we could help those who are in difficulty.______
3. This is the river which I used to swim when I was a student.______
4. This is one of the best film I have ever seen.______
5. China now can produce the fastest computer of modern time.______