用can,must,can't,mustn't填空
1.The children ____ play on the road.It's very dangerous.
2.---____ you skate on real ice?
---Yes,I can.It's easy.
3.---Who's that girl?
---It ____ be Mary.Mary has gone to Beijing.
4.As students,we ____ work hard at school.
5.---____ I clean the classroom now?
---No,you needn't