单选题
A decorator bought a bolt of defective cloth that he judged to be 
usable, in which ease the cost would be $0.80 per usable yard. If it was later found that only 
usable, in which ease the cost would be $0.80 per usable yard. If it was later found that only
【正确答案】
B
【答案解析】[解析] 设这匹布共有a码，每码可以用的布的实际成本为x美元，则根据题意可得：
