When asked why he enjoys rock climbing so much,Bibby said,"It's basically like dancing on the rock.

It's a challenge________man against nature."[    ]

A. of

B. for

C. by

D. over

一国本币贬值有于利于该国进口，不利于该国出口；一国本币升值有于利于该国出口，不利于该国进口。