We can infer from the passage that the oldest form of communication between people is ______.

A．Humans do not have adequate knowledge about animal languages.
B．We are now very much aware of animal languages.
C．Most of animals use their facial expressions.
D．All the animals make sounds to communicate with each other.

A．all advertisements of a certain type are grouped together
B．it is more distinguished
C．it is more expensive
D．nowadays the classified advertisements are all of the same size