Studio 23 Salon sells a popular brand of nail polish to its clients for $11 a bottle. The salon pays $6 a bottle for this nail polish if it buys fewer than a dozen bottles at a time. It pays $4 per bottle for the nail polish if it buys more than a dozen bottles at a time. What is Studio 23 's per bottle markup on the nail polish if it buys 18 bottles?

a $11
b $4
c $7
d $5