单项选择题 The carbon deposits on the piston crown and in the ring grooves （）.

单项选择题 The cap and new shell are tightened up and then removed and the thickness of the wire measured by （）.

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?（）