The increasing move to the cities of the people is thought to be （）

A．since the last century
B．for a very long time
C．since a few hundred years ago
D．since a few years ago

A．When language is a barrier, people will find other forms of communication.
B．Everybody uses only one form of communication.
C．Nonlinguistic language is invaluable to foreigners.
D．Although other forms of communication exist, verbalization is the fastest.