study的過去式為什么把y去掉

　　Many toxic effects can be studied at the cellular level.

　　許多毒效可以從細胞層次上去研究。

　　She studied chemistry for three years.

　　她學了三年化學。

　　I studied English and Drama at college.

　　我在大學學的是英語和戲劇。

　　My brother studied at the Royal College of Art.

　　我哥哥曾就學于皇家美術學院。

　　The report found that 30% of the firms studied had failed within a year.

　　據報告稱，受調查的公司有30%一年內倒閉了。