问题
选择题
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
题目分析：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。