People           about what our future will look like for thousands of years.

have been wondering

wonder

wondered

will wonder

考查动词时态。句意：数千年以来人们一直想知道我们未来是什么样子。根据时间状语 for thousands of years可知，描述从过去开始一直持续到现在的动作，用现在完成进行时。故选A项。

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。

---Where is Class       ?

---It’s on the       floor.

Six; third

Sixth; third

Six; three

Sixth; three

考查数词辨析。句意：----六班在哪里？---在三楼。英语中表达序号时，如果用基数词表示：名词+基数词，如六班，Class Six;如果用序数词表示：the+序数词+名词，如三楼，即the third floor。故选A项。

______about wild plants that they decided to make a trip to Madagascar for further research.

So curious the couple was

So curious were the couple

How curious the couple were

The couple was such curious

I thought we'd be late for the concert,           we ended up getting there ahead of time.

but

or

so

for

考查连词。句意：我以为我们音乐会迟到了，但是我们提前到达了那里。but 但是；or 或者；so 因此；for因为；两个分句在意义上bu是转折关系，故用but。故选A项。