________ no one wants to read the book, they have to read it.   

A．So

B．Although

C．And

D．But

[问题1] 请在下列选项中选择合适的答案，填入图3-1、图3-2的方框a和方框b。 B的公钥，B的私钥，摘要算法，A的私钥，A的公钥，会话密钥