Choose the word or phrase ( A, B, C or D ) that best fits the blank space in the following passage.
Speech is one of the most important ways of communicating. It consists of far more than just making noises. To talk or to ................by other people, we have to master a language, that is, we have to use combinations of sound that stand for a particular object or idea. Communication is impossible if everyone makes up their own language.
be understood understanding understand be understandable