He spent a lot of time____about engines(发动机).

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He spent a lot of time____about engines(发动机).

He spent a lot of time____about engines(发动机).
He spent a lot of time____about engines(发动机).

He spent a lot of time____about engines(发动机).
可以填studying.
答案有很多,反正必须是动词ing形式.

on sth
in doing sth

researching