They were dancing when they heard the bell的问句是什么?

What were they doing when they heard the bell?（问他们在干吗）
或者：
What happened when they were dancing?（问他们跳舞时发生了什么）
What did they hear when they were dancing?（问他们跳舞时听到了什么）
What did they hear while dancing?（同上）
总之要根据问话人的目的.