求讲一道很简单的题..宾语从句的
——Do you know __________Mr Black's address is?
——He may live at NO.18 or NO.19 of Bridge Street. I'm not sure of______
A.where,which B.where,what C.what,which D.what,where
谁能告诉我为什么选 C ,第一个空为什么用what不用where.谢谢.