A、Making noises. B、Our own system of language. C、Combinations of different sounds. D、A group of sentences upon our own creations

Speech is one of the most important ways of communicating. It consists of far more than just making noises. To talk and also to be understood by other people, we have to speak a language, that is, we have to use combinations of sounds that everyone agrees stand for a particular object or idea. Communication would be impossible if everyone made up their own language.
    Learning a language properly is very important. The basic vocabulary of English is not very large, and only about 2,000 words are needed to speak it quite well. But the more words you know, the more ideas you can express, and the more precise you can be about their exact meaning.
    Words are the main thing we use in communicating what we want to say. The way we say the words is also very important. Our tone of voice can express many emotions and shows whether we are pleased or angry, for instance.
19. What are used to stand for a particular object or idea when we speak a language?
20. What if everyone made up their own language?
21. What is also important besides the main thing in communicating?