study的过去式为什么把y去掉

Many toxic effects can be studied at the cellular level.

许多毒效可以从细胞层次上去研究。

She studied chemistry for three years.

她学了三年化学。

I studied English and Drama at college.

我在大学学的是英语和戏剧。

My brother studied at the Royal College of Art.

我哥哥曾就学于皇家美术学院。

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

据报告称，受调查的公司有30%一年内倒闭了。