Sweaters sell for $15 at the crafts fair. Allie knits sweaters and her marginal costs are given in the table below. Allie's marginal costs increase with each additional sweater. If Allie is behaving rationally, how many sweaters will she sell?

a. 1st- 5$
b. 2nd- 8$
c. 3rd- 12$
d.4th- 18$
e. 5th- 25$
f. 6th- 32$