study的過去式爲什麼把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.

我哥哥曾就學於皇家美術學院。

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

報告稱,受調查的公司有30%一年內倒閉了。