单项选择题 The burning of the fuel adds more heat to the air charge, causing it to expand and force the engine piston to do work on the crankshaftWhat does “it” mean in this sentence?（）

单项选择题 The bore of each cylinder is formed in a liner which can be （） when worn out.

单项选择题 The bed-plate is secured in the longitudinal direction by means of bolts known as （）.