______ (开始的时候) I thought it very difficult to study English.

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　　A. On first   B. At first   C. First   D. At begin

开始下雨了

It started ________ ________.

开始下雨了

It started ________ ________.

开始，他们彼此不认识。

They didn't know each other ________ ________ ________.

开始，我发现这事很难。

In the beginning, I found ________ ________ ________.