单选题
A certain alloy contains only lead, copper, and tin. How many pounds of tin are contained in 56 pounds of the alloy?
(1)By weight the alloy is
lead and 
(1)By weight the alloy is
lead and
【正确答案】
A
【答案解析】[解析] 本题的正确答案为(A) 。由(1)中的铅在该合金中所占的比例等于
，铜的比例等于
可推知锡所占的比例等于
，用锡的总量56乘以所占的比例
，铜的比例等于可推知锡所占的比例等于，用锡的总量56乘以所占的比例