How to use the "that,which,what"in sentence construction?我总是分不清that，which，what在从句时，该用哪个？ eg：I know the city that(where?) in the east of China? use that?or use where?maybe use which? eg：I know the city that(where,which) in the east of China. ----------------I am not sure ,does it use that?or use where?maybe use which?