重点讲解：
1. ask sb. to do sth. 要求…（做…）；请求…（做…）；
Homework assignments and examinations will often ask you to explain something or to define something that you have been taught.
在作业和考试过程中，常常会要求你解释或定义一些你已经学过的事物Il_P(zaS,Gg。
2. be fluent in 流利的；流畅的；
She studied eight foreign languages but is fluent in only six of them.
她学习了8种外语，但是只能流利地说其中 6 种h4&A!k3(.QqHjs。
3. focus on 集中(注意力)；关注；
In this article, we focus on encryption and digital signatures.
本文中我们主要讨论加密和数字签名2p5(tLT*vTCG。