I bought a dress for only 10 dollars in a sale; it was a real _______.
A.exchangeB.bargainC.tradeD.business在线课程B
解析：
此处考查名词的用法。bargain意为“便宜货，廉价货”。exchange意为“交换，互换”；
trade意为“贸易”；
business意为“商业”，不符题意。故选B。