单项选择题
Smaller engines can be made more rigid than large ones and may not have （）.
A.connecting rods B.crankshaft C.crankcase D.through bolts
单项选择题 Since the burning or combustion takes place within the engine itself, the engine is known as （）.
单项选择题 Shims of different thicknesses are inserted between the shells to enable adjustment of （）.
单项选择题 Sediment collects in a tubular purifier （）.
