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

A．They have their advanced language in form of sound.
B．They can express simple feelings such as hunger and pain.
C．Some of them can communicate with people very well.
D．All of them can use sound to express themselves.

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