Identify the revised sentence that eliminates the pronoun reference problem if the following sentence.

I think the professor covered it in a lecture, but I don't really remember how it works.

I think he covered it in a lecture, but I don't really remember how it works.

I think the professor covered mitosis in a lecture, but I don't really remember how it works.

I think the professor covered that in a lecture, but I don't really remember how it works.

I think the professor covered it in a lecture, but I don't really remember how this works.

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