单句改错。

1. Your friend comes to school is very upset. _________

2. Tell your friend that you are concerning about him. _________

3. While walk the dog, you were careless and it got loose. __________

4. Join the examination and you'll get a chance. __________

5. Yesterday I got my bike repair. __________

有一辆汽车以83000元购入，实行加速折旧，每年的运行维持费用和折旧后每年残余价值如表所示。试计算其最佳更新期。