Jon Brooks quit his job in a bicycle shop, where he earned $15,000 per year, to become a graduate student in economics. At the university he attended, he spent $2,000 on books, $1,000 on cough medicine, and earned $12,000 as an economics instructor. What were Jon's economic costs while attending college?
A. $18,000
B. $15,000
C. $6,000
D. $3,000