The passage tells us that the best time for a child to learn languages is （）

A．the phenomenon of birds dozing in pairs is widespread
B．birds prefer to sleep in pairs for the sake of security
C．even an imagined companion gives the bird a sense of security
D．a single pet bird enjoys seeing its own reflection in the mirror

A．The world has to face the fact that it is running out of space to put its trash.
B．Having no space to dump the garbage forces some countries to begin recycling programs.
C．Recycling was once unfavorably considered as a counter culture activity.
D．We have to admit that recycling of the garbage will never move into the mainstream.