A large bottle of 1200ml drink costs 8 yuan, a small bottle of 400ml costs 3 yuan, store A buys a large bottle and gets a small bottle free, store B sells a 10% discount, and store C sells a 15% discount for a purchase of 30 yuan. Which store is the cheapest to buy 12 liters?
大瓶饮料1200毫升，卖8元，小瓶400毫升卖3元，甲店买一大瓶送一小瓶，乙店九折销售，丙店满30元8.5折，问买12升哪个店最便宜？
<result>12</result>