When do children first begin to make language-like noises

A．It made the work of professionals more enjoyable.
B．It made picture-taking suddenly popular among ordinary people.
C．It marked the beginning of manufacturing small cameras.
D．It met the need of photographers in America and Europe.

A．is something that is best illustrated in animal books.
B．is an accepted standard of overpopulation.
C．has been the subject of many good stories.
D．is a very unclear concept.