【题目】By the time he was fifteen years old, he ________ advanced maths all by himself.
A. had learned B. have learned
C. learned D. would learn
在线课程【答案】A
【解析】考查时态。句意：到他15岁为止，他已经自学完了高等数学。by the time引导时间状语从句，主句则表示在此时间之前某个事件已经完成。当从句用过去时，主句通常用过去完成时。由句意可知，在15岁之前，他已经自学了高等数学。故该空应用过去完成时态。