The world is changing fast.I strongly believe that one will easily fall behind he keeps o

问题补充：

The world is changing fast.I strongly believe that one will easily fall behind he keeps on learning.

A. unless

B. if

C. when

D. as

答案：

(答案→)A. unless

B. if

C. when

D. as

解析：本题考查连词用法。根据句意，如果不学习很容易落下；所以选A项。