Tại sao chỗ trống dùng That mà không dùng Which?

Speech is one of the most important way of communicating .It consists of far more than just making noises . To talk and also be understood to by other people, we have to speak a language ,that is, we have to use combinations of sounds ___ everyone agrees to stand for a particular object or idea. Communication would be impossible if everyone made up their own language.
Mọi người ơi cho mình hỏi là chỗ để trống đó mình thấy là đề đều giải là dùng that tại sao không dùng which ạ