1. Doing sit-ups is is quite simple.

Question

2. It is quite simple doing sit-ups.
3. To do sit-ups is quite simple.
4. It is quite simple to do sit-ups.
[Can we use all the expressions? Are they all the same? Which ones do you use more often?]

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Answer 1

I wouldn't use 2.

The others are fine, although 3 is rather awkward.