They are having a (Chtirstmas)party tomorrow.

问题描述:

They are having a (Chtirstmas)party tomorrow.
对括号内部分提问
___ ___ ___ they having tomorrow?

what party are
我觉得用what 比which好
因为提问的人并没有一个可以选择的范围
若有范围,用which
否则就是what