study的過(guò)去式和過(guò)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(xué)習(xí)過(guò)。

　　He studied conscientiously and enthusiastically.

　　他認(rèn)真積極地學(xué)習(xí)。

　　Many toxic effects can be studied at the cellular level.

　　許多毒性作用在細(xì)胞階段就可加以研究。