有读者问： 有这样一道选择题：
(1)— When can I come for photos? I need them tomorrow afternoon.
— They_________be ready by 12:00.
A. can B. should
C. might D. need
答案是B，请问A为何不可？
情态动词should在此表示“可能”，具有推定性，无十分把握，相当于汉语的“料想”，再如：
(2)She should be here in a minute.
(3)That should be easy.
(4)Three weeks should suffice.
(5)You should be able to learn to swim in three hours.
情态动词can表理论上“可能”，如：
(6)Lightning can be dangerous.
(7)He can sometimes be very nasty.
因此此处不宜用can。
