How men first learnt to invent words is unknown; in other words, the origin of langua

ge is a mystery. All we really know is that men, unlike animals, somehow invented certain sounds to express thought and feelings, actions and things, so that they could communicate with each other; and that later they agreed upon certain signs, called letters, which could be combined to present those sounds, and which could be written down. Those sounds, whether spoken or written in letters, we call words. The power of words, then, lies in their associations - the things they bring up before our minds. Words become filled with meaning for us by experience; and the longer we live, the more certain words recall to us the glad and sad events of our past; and the more we read and learn, the more the number of words that mean something increases. Great writers are those who not only have great thoughts but also express these thoughts in words which appeal powerfully to our minds and emotions. This charming and telling use of words is what we call literary style. Above all, the real poet is a master of words. He can convey his meaning in words which sing like music and which by their position and association can move men to tears. We should therefore learn to choose our words carefully and use them accurately, or they will make our speech silly and vulgar. What is true about words？

：A、They are used to express feelings only

B、They can not be written down

C、They are simply sounds

D、They are mysterious.