research 的过去式是?

resear-researched（过去式）-researched（过去分词）

research
KK: []
DJ: []
n.
1. (学术)研究;调查,探究[S][P1][(+in/into/on)]
She's doing cancer research.
她在从事癌症研究。
vi.
1. 作学术研究;调查,探究[(+in/into/on)]
He is researching into the causes of cancer.
他正在研究癌症的起因。
vt.
1. 研究,探究
The historical background to the play has been very carefully researched.
这部剧本的历史背景已有人作过仔细的研究。