all you need to do is find the gradient of the line after the point 30000. so you would take the point (30000,1000) and (40000,3000) and do rise over run to get your gradient i.e:
the rise is 2000 (3000-1000) and the run 10000 (40000-30000)
therefore 2000/10000 = 0.2 or 20cents
so c is the answer