— I am afraid I can't return the book to you before Friday. 

— _____. [ ]

A. Don't be afraid

B. Be careful

C. Not at all

D. Take your time

从传统意义上来说，研究（）的科学与技术被称为密码学。

A、信息加密和解密

B、软件加密与解密

C、密码设置和保存

D、破译密电码