单句改错（共5小题，每小题1分，共 5分。每句只能改一处,在原句中直接改正）
Air hostesses all smile when they serve for the passengers.
No one matches him when it comes to swim.
A poor nourished body needs more energy to combat disease.
Foreigners always associate China to the Great Wall.
Not until then he realize the importance of the problem.