-How amazing it is that astronauts are exploring outer space!--It’s a challenge I

问题补充：

---How amazing it is that astronauts are exploring outer space!

----- It’s a challenge, I guess, man against nature.

A. of

B. for

C. by

D. about

答案：

(答案→)A. of

B. for

C. by

D. about

解析：考查介词。应答句中的I guess是插入语，因此所填介词与challenge连用，表所属，用of，选A。