It’s a difficult skill to master but it will benefit you for the rest of your life. A.

问题补充：

It’s a difficult skill to master, but , it will benefit you for the rest of your life.

A. once obtained

B. when to obtain

C. after obtaining

D. while obtaining

答案：

(答案→)A. once obtained

B. when to obtain

C. after obtaining

D. while obtaining

解析：考查非谓语动词。句中的a difficult skill与obtain之间为被动关系，故用过去分词。句意：它是个很难掌握的技巧，但一旦被掌握，将对你的余生有好处。故A正确。