The things brought down by rivers from the mountains that are useful for cultivation are （）.

A. Can I help you
C. Safety belt, please.
E. Is there anything serious
G. What flight are you taking
B. We are taking off at last.
D. The engine went wrong.
F. Just a moment, please.
H. It should be OK in an hour.

A．they know nothing about the world
B．they know little about the world
C．there are answers they do not have
D．all of the above