Which is the proper explanation of the word "fade'(L. 2, para. 3 )

A．A bird learns to sing and fly after twelve weeks.
B．A bird learns to sing and fly when it is old enough.
C．An infant learns to smile and make vowel-like sounds at twelve weeks.
D．A bird learns to sing and fly fast at a certain stage.

A．they are unable to learn language rapidly
B．their mothers are not clever about a child
C．their mothers respond inadequately to their attempts to speak
D．they are exposed to too much language at once