求助一题，没找着

In the United States, vacationers account for more than half of all wisitors to waht are technically called "pure aquariums" but  for fewer tahn one quarter of all visitors to zoos, which usually include a "zoo aquarium" of relatively modest scope.   Which of the following, if true, most helps to account for the difference described above between visitors to zoos and visitors to pure aquariums?

A    In cities that have both a zoo and a pure aquarium, local residents are twice as likely to visit the aquarium as they are to visit the zoo.

B     Virtually all large metropolitian areas have zoos, wheres only a few large metropolitan areas have pure aquariums.

C   Over the last ten years, newly constructed pure aquariums have outnumber newly established zoos by a factor of two to one.

D  People who visit a zoo in a given year are two times more likely to visit a pure aquarium that year are people who do not visit a zoo.

E  The zoo aquariums of zoos that are in the same city as a pure aquarium tend to be smaller than the aquariums of zoos that have no pure aquarium nearby.

答案是B  我选了C，为啥啊。    想得头都长毛了。~~~~~~~~~~