A library argues: “Extending our opening hours into the evening will increase visits.” Which assumption is necessary for that prediction?

A little clarity goes a long way.

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  A (All current visitors will stop reading at home.):

  The prediction does not require all visitors to change their reading habits; this is far stronger than necessary.
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  B (The library will buy no new books.):

  Book purchases are not required or excluded by the argument about opening hours.
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  C (Some people want to visit during the added hours and their visits will not merely replace existing visits.):

  Without some additional demand that produces net new visits, longer hours alone do not imply an increase in total visits.
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  D (Every resident already owns a library card.):

  Potential new visitors could obtain cards later, so universal current membership is unnecessary.