gmat逻辑问题

gmat逻辑问题
7.Fresh potatoes generally cost about $2 for a 10-pound bag,whereas dehydrated instant potatoes cost,on average,about $3 per pound.It can be concluded that some consumers will pay 15 times as much for convenience,since sales of this convenience food continue to rise.
Which of the following if true,indicates that there is a major flaw in the argument above?
(A) Fresh potatoes bought in convenient 2-pound bags are about $1 a bag,or 2 1/2 times more expensive than fresh potatoes bought in a 10-pound bags.
(B) Since fresh potatoes are 80 percent water,one pound of dehydrated potatoes is the equivalent of 5 pounds of fresh potatoes.
(C) Peeled potatoes in cans are also more expensive than the less convenient fresh potatoes.
(D) Retail prices of dehydrated potatoes have declined by 20 percent since 1960 to the current level of about $3 a pound
(E) As a consequence of labor and processing costs all convenience foods cost more than the basic foods from which they are derived.
为啥选B不选A