问题补充：
Can you tell me at that time?
A. what were they doing
B. what they were doing
C. what are they doing
D. what they are doing
答案：
A. what were they doing
(答案→)B. what they were doing
C. what are they doing
D. what they are doing
解析：句意：你能告诉我那个时间他们正在做什么吗？宾语从句的结构：主语+谓语+连接词+从句。从句在句中作宾语。从句要用陈述句的语序。如果主句是一般现在时，从句根据需要选择时态，如果主句是过去时态，从句要用相应的过去时态。主句是一般现在时，从句根据需要用过去进行时。AC语序错误，D时态错误，故选B
