According to the passage we know tipping cannot be explained ______.

A．It is to reward the efforts of good service.
B．It is a promise of on-time service.
C．It is used to reduce uncomfortable feelings of inequality.
D．It serves no useful function.

A．Peter cannot wait to attend the course.
B．At first he was reluctant and timid.
C．Mike enjoyed the open-air life very much.
D．Peter did not get well along with his classmates.