单选题 I was listening to the radio ______. I heard a knock at the door.
A. while B. when C. as

【正确答案】 B

【答案解析】[详细解答] 此题考查连词。本句意为：我正在听收音机，就在这时有人敲门。when意为就在那时，相当于at that time。