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

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(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。