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?（）

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?（） A: the burning B: the fuel C: the air charge D: the engine pisto