Smaller engines can be made more rigid than large ones and may not have （）.

A.a fire -tube boiler
B.an internal combustion engine
C.an external combustion engine
D.an internal combustion generator

A.the bearing clearances
B.the bearing thickness
C.the bearing strength
D.the bridge hardness

A.on top cover
B.on sides of bowl
C.in drain line on the discs
D.on the discs