在表语从句中,如果先行词在从句中担当主语或宾语,连接词用什么?是不是只能用what?

抱歉，表语从句里好像没有先行词……
整个表语从句做表语……
如：
The question is when he can arrive at the hotel.
“when he can arrive at the hotel”为从句部分，作表语。
连接词：that, what, who, when, where, which, why, whether, how

that which 也可以

连接代词：who,whom,that,which;
连接副词：where,when,how,why;
连接词：what,whether.
均可以连接表语从句.