A store offers a discount if a customer purchases shirts in bulk. The table shows the relationship between the cost (c) in dollars per shirt and the number (s) of shirts purchased.

s 3,6,9,12

c 9.5,9,8.5,8

Which equation represents these data algebraically?

a. c= 10-s/6

b. c=15-s

c. c=s-0.5

d. c= 3s/2