用所给词的适当形式填空。

l. Don't play while _____ (eat).

2. It's_____ (usual) for a 7-year-old boy to work out the difficult problem.

3.Who jumps_____ (high)in your class?

4. Why don't you try_____ (ride) a bike to go to school?

5. Keep_____ (work) hard until you make it.

撤销要约的通知，应当在( )到达受要约人。

A．要约到达之前

B．与要约同时

C．受要约人发出承诺通知之前

D．受要约人发出承诺之后