一. 过去进行时：
( )1. My best friend visited my house while I _______ dinner in the kitchen.
A. was cooking B. cooking C. cooked
( )2. My mother was cooking while I ____ the radio.
A . listened to B. have listened to
C. is listening to D. was listening to
( )3. He was sleeping late _______ someone knocked on the door.
A. when B. while C. as D. after
( )4._____ I was talking on the telephone, David came in.
A. Because B. While C. If D. When
