Solid fuel rockets are expensive to operate because of their ______.

A．whether a liquid-fuel or a solid-fuel rocket is better depends on the purpose
B．neither type is superior
C．forty years ago, large solid-fuel rockets with solid fuels as powerful as liquid fuels were made
D．the thrust can be made to vary by adjusting the direction of the pump

A．inexpensive to construct
B．capable of lifting heavy spacecraft into orbit
C．easily controlled
D．inexpensive to operate