study为什么不变y为i加ing

　　例句：

　　He's studying music in his spare time.

　　他在空闲时间学音乐。

　　I am studying Chinese.

　　我正在学中文。

　　The students were questioned on the books they had been studying.

　　学生被问到有关他们所学课本内容的问题。

　　She's studying art history.

　　她正在研读艺术史。