A certain alloy contains only lead, copper, adn tin. How many pounds of tin are contained in 56 pounds of the alloy?
(1) By weight, teh alloy is 3/7 lead and 5/14 copper.
(2) By weight the alloy contains 6 parts lead and 5 parts copper.
正确答案是A。(2)中的parts是什么意思?