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．the index of one's status in a sense
B．part of the accepted cost of a service
C．a reflection of their character
D．a kind of personal attitude