Emotional "roller coaster" refers to____________________.

A．Warm-hearted but careless.
B．Curious and mischievous.
C．Helpful and tricky.
D．Prudent but cautious.

A．Because there would be less damage when the plane explodes under water,
B．Because the pressure of water is always smaller than that of air.
C．Because this is the only way to find out which part has cracked.
D．Because some parts of a plane are more likely to crack in water than in the air.