The amount of sales of single scoop ice-cream cones during the summer s(c) varies directly with the number of customers c. Twelve customers bought a total of $54 worth of ice-cream cones. Which equation can be used to find the ice-cream sales for c customers?
a) s(c) = 4.5c
b) s(c) = 6c
c) s(c) = 3c
d) s(c) = 2.5c