study的過去式和過去分詞怎么寫

　　Coincidentally, they had both studied in Paris.

　　碰巧的是，他們倆都在巴黎學習過。

　　He studied conscientiously and enthusiastically.

　　他認真積極地學習。

　　Many toxic effects can be studied at the cellular level.

　　許多毒性作用在細胞階段就可加以研究。