In secondary school classes, discussions are different from the question-and-answer sequences because discussions require ______.

A．He knows less about concepts of engineering.
B．He can’t explain some concepts of engineering.
C．He gives talks to high school students regularly.
D．He doesn’t want the woman to postpone the talk.

A．Only if we have much in common.
B．Only if we know our friends’ mistakes.
C．Only if we treat our disagreements wisely.
D．Only if we have known one another for long.