As used in the first paragraph, terrain most clearly means ______.

A．The computer simulates rays of the sun, filling in areas of light and shadow.
B．Lines radiate outward from the imagined observer and a dot of color is placed where the line intersects with one of the points of altitude in the machine's memory.
C．X-rays are used to trace the outline of the terrain through buildings and trees.
D．The exact movement of rays is used by private detectives to solve mysteries and locate missing persons.

A．neither type is very economical
B．the liquid-fuel rocket is best
C．each type has certain advantages
D．the solid-fuel rocket is hest