According to the rules of the library you "must" keep the books for a fortnight.
Объяснение: b) must
According to the rules of the library you may keep the books for a fortnight
То есть, данный ответ: may (a)
According to the rules of the library you "must" keep the books for a fortnight.
Объяснение: b) must
According to the rules of the library you may keep the books for a fortnight
То есть, данный ответ: may (a)