数字签名和手写签名的区别是________。

A．前者因消息而异，后者因签名者而异

B．前者因签名者而异，后者因消息而异

C．前者是0和1的数字串，后者是模拟图形

D．前者是模拟图形，后者是0和1的数字串

— There _____ be anybody in the classroom, for the lights are off.

— I'll go and check. [ ]

A. mustn't

B. won't

C. needn't

D. can't