我想，这是人类与大自然搏斗的主要难题之一。

challenge常与of连用，of 表示“coming from”来源于。

3 Children need friends________their own age to play with.

A.of

B.for

C.in

D.at

答案选A，儿童需要和他们同龄的孩子一起玩。of one's own age.与他们同龄的，介词短语作后置定语。

4 China has got a good ______for fighting against the flu with its careful and smooth organization.

A.reputation

B.influence

C.impression

D.knowledge

答案选A， 在抗击流感方面，中国以细致和流畅的组织而获得良好的声誉。influence影响 impression 印象

5 In our class, when the bell rang and the teacher closed his book,

it was a_______for everyoneto stand up.

A. signal

B.chance

C. mark

D. measure

答案选：C。

在我们的课上，当上课铃响，教师合上书就是我们要起立的信号。

A. signal信号，导火线

B.chance机会，运气

C. mark分数，标志

D. measure措施，办法，尺寸

6 Four Chinese models were _________the 14 people awarded prizes on Friday at the World Supermodel Competition.

A.among

B.between

C.along