1.      When filled with a gas lighter than air,a balloon can_____in the air. A.fly　 B.float　 C.wander　 D.sail

[　　]

A．When filled 　　　B．Filling

C．Having filled 　　D．When filling

________ with a gas which is lighter than air, a balloon can float in the air.

1. A.
   When filling
2. B.
   When filled
3. C.
   Filling
4. D.
   Having filled

________ with a gas which is lighter than air, a balloon can float in the air.

[　　]

A．When filling　　　　　　B．When filled

C．Filling　　　　　　　　 D．Having filled

________ with a gas which is lighter than air, a balloon can float in the air.

[　　]

A．When filling　　　　　　B．When filled

C．Filling　　　　　　　　 D．Having filled