Questions 19 to 21 are based on the conversation you have just heard.

A. Diamond-producing rivers are located far away the mountain side.
B. Diamonds can be formed without volcano heat and pressure.
C. Volcano explosions brought some diamonds up to the earth surface.
D. Explosions of the volcano can damage diamonds as well.

A. Because they lack self-discipline in their studies.
B. Because they spend too much time on the Internet.
C. Because they have not exerted their utmost efforts.
D. Because they have developed poor relationships with teachers.