Bicycle rental on the seawall at the beach cost a mandatory minimum of $5 for the first hour and then $4 for each additional hour. when three or more bikes are rented together, the owner gives a 20% discount on the additional hours. if marty and his family rent four bicycles, identify the expression which represents the cost.

a) 12.8x + 20x
b) 12.8x + 20
c) 12.8x + 16x
d) 32.80x