Cody summer sells tires for tractor trailers. he earns 4.5% commission on the first $6,000 in sales, 8% commission on the next $6,000, and 12% commission on sales over $12,000. last month he sold $18,000 worth of tires. what was his total commission?
a) $1,540
b) $1,470
c) $1,450
d) $1,650