It was last night ________ I saw the comet.

the time

when

which

强调句的结构是：Itbe强调部分that(who)主谓句。强调句的连词只有两个，that和who。当强调的部分是人，且为句子的主语时，才用“who”，其余用that。因此选C。

When it comes to modern communication,network safety is always a major.

concern

advantage

opportunity

solution

考查名词辨析。句意：当涉及现代通信，网络安全一直是一个主要令人担忧的问题。A. concern关心，令人担忧的事实或情况；B. advantage 优势；C. opportunity 机会；D.solution解决方案。网络安全一直是现代通信的个主要令人担忧的问题。故选A项。

If you work hard like this,you will sooner or later.

give out

break up

break down

fall down

考查动词短语。句意：如果你继续这样辛苦工作的话，你迟早垮掉。A.give out分发；B.break up打碎；C.break down垮掉；D.fall down 跌倒。根据句意可知break down符合语境。故选C项。

It was along the Mississippi River _________ Mark Twain spent much of his childhood.

how

which

that

where

此处构成It was…that…强调句式，句中强调了介词短语along the Mississippi River。故选C。