Answer:
g
Explanation:
for this function, we put in the value of "g" (because it is the cause of the function--it is the independent variable [it changes; and it doesn't rely on something else to change] )
so, because the part independently changing is the gas used, that is our input of this graph.
(you can imagine this to be a graph, which typically has the independent variable that we are directly changing--x--along the x-axis, which is where we would place g.)
hope this helps!!