Like other animals, we are          until we know it is safe to relax.

on purpose

on fire

on guard

on duty

考查介词短语辨析。句意：像其他动物一样，我们在知道安全的时候才放松警惕。A.on purpose 故意；B. on fire着火；C.on guard 警惕；D.on duty值班。结合后文we know it is safe to relax，可知我们在知道安全的时候才放松警惕。故选C项。

He          and wept when he heard the news.

took down

put down

turned down

broke down

考查动词短语辨析。句意：当他听到这个消息时，整个人都崩溃了并哭起来。A. took down  记下；B.put down记下；C.turned down调小，拒绝；D.broke down崩溃，坏掉。根据wept 可知此处应填"broke down"符合语境。故选D项。

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。

 Is _______ three hours _______ the poor boy to come to school on foot?

it; that it takes

there; that it takes

there; when

it; that

考查强调句。强调句的结构为It’s + 强调部分 + that从句。本题强调时间状语three hours。句意：可怜的男孩步行到学校花了3个小时吗？故A正确。

--- I'm afraid I can't complete the marathon next week.

---              ! You have been practicing a lot.

No problem

Go ahead

Cheer up

Good luck

考查交际用语。句意：---恐怕我下周不能完成马拉松比赛。---振作起来！你一直一直在不断练习。A.No problem没问题；B. Go  ahead请干吧；请便吧；C.Cheer up高兴起来；振作起来；D. Good luck祝您好运。由下文可知说话人担心，完不成马拉松比赛，由“你一直不断练习”可推断表示鼓励“振作起来”。故选C项。