Regardless of Area of Expertise,a Public School Pays All Teachers

Regardless of area 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?

A) $90,000
B) $100,000
C) $20,000
D) $450,000