How do you get 90? Please explain

Security A pays $100 in a year, and $100 in two years. The price of security A is $170. Security B pays $100 in two years. The price of security B is $80. Security C pays $100 in a year. What must be the price of security C? Select one: O a. 70 O b. 80 O c. 100 O d. 90 ~ Check