Regardless of expertise, a public school pays all teachers the same annual wage. At $70,000 it hires good teachers in all areas except science. Because science is a field in demand, the school can only hire poor science teachers. It would have to pay science teachers $90,000 to hire good science teachers. If one out of five teachers is a science teacher, what is the marginal expense of getting a good science teacher instead of a poor one?

-$90,000

-$100,000

-$20,000 

-$450,000

If possible, explain your answer and tell me what your credentials regarding economics are.