In a room above the store,where a party
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some workers were busy setting the table
这是主句.所以前面都是非谓语形式.
B.was to be held
A.has been held
这都是谓语,不行.
In a room above the store,where a party__being held_,
======以下答案可供参考======
供参考答案1:
我觉得b也可以
供参考答案2:
正在收拾桌子是过去时，聚会是在收拾桌子以前，应用过去完成时。