5.In the United States， vacationers account for more than half of all visitors to what are technically called “pure aquariums” but for fewer than 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 metropolitan areas have zoos，whereas only a few large metropolitan areas have pure aquariums.

　　（C） Over the last ten years，newly constructed pure aquariums have outnumbered 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 than are people who do not visit a zoo.

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

　　6.In the United States， the Postal Service has a monopoly on first-class mail， but much of what is sent first class could be transmitted electronically. Electronic transmittal operators argue that if the Postal Service were to offer electronic transmission， it would have an unfair advantage， since its electronic transmission service could be subsidized from the profits of the monopoly.

　　Which of the following， if each is true， would allay the electronic transmittal operators' fears of unfair competition？

　　（A） If the Postal Service were to offer electronic transmission， it could not make a profit on first-class mail.

　　（B） If the Postal Service were to offer electronic transmission， it would have a monopoly on that kind of service.

　　（C） Much of the material that is now sent by first-class mail could be delivered much faster by special package couriers， but is not sent that way because of cost.

　　（D） There is no economy of scale in electronic transmission——that is，the cost per transaction does not go down as more pieces of information are transmitted.

　　（E） Electronic transmission will never be cost-effective for material not sent by first-class mail such as newspaper and bulk mail.