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。

There's no simple solution          this problem.

of

for

to 

about

考查介词。句意：这个问题没有什么简单的解决方法。此处是短语“

the solution to....的解决方法”，应用介词to。故选C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。

Perhaps they have experienced something similar and do not want you to           the same pain.

go over

go through

go around

go on

考查动词短语辨析。句意：也许他们有过类似的经历，不希望你经历同样的痛苦。A. go over复习，重温；B. go through经受，通过；C. go around 四处走动，流传；D. go on继续。根据the same pain可知，此处是“经历，经受”的意思。故选B项。

The old rules have to be          because they only applied to the circumstances that existed when they were made a hundred years ago.

developed

established

observed

revised

考查动词的辨析。句意：老的规则需要被修改因为它们只适合一百年制定时的状况。A.develop发展；培养；B.establish建立；C.observe观察；D.revise修改，由because they only applied to 可知它只适用于以前，需要修改。故选D项。

It is well-known that crack cocaine is a powerful             drug and users become           to it much more easily if they smoke it.

addicted; addict

addictive; addict

addicted; addictive

addictive; addicted

考查形容词辨析。众所周知可卡因是一种强大的令人上瘾的毒品，如果使用者吸了它更容易上瘾得多。addicted上瘾的，着迷的；addict"shi使沉迷；使上瘾”是动词；addictive “使成瘾的，令人沉迷的”。第一空表示“令人上瘾的”用addictive，故排除A，C项；become addicted to “对....上瘾”，第二空填addicted。故选D项。

—Have you watered the flowers?

I am

I’m going

I’m just going to

I will go

句意为：——你浇花了吗？——没有，不过我正打算浇。I’m just going to.相当于I’m just going to water the flowers。故选C。

Since this accident has nothing to do with him, he seems no way             .

to be blamed

to blame

blame for

to blame for

考查固定用法。句意：既然这次事故与他无关，他似乎也就无可指责了。短语sb to be blame“某人该受责备，某人应该负责”，为固定搭配，没有被动语态。故选B项。