"Sorry to have kept you waiting" vs. "sorry for having kept you waiting"

grammaticality

Solution 1:

I am sorry to have kept you waiting so long. It is fine but can be a bit better:

I am sorry to have kept you waiting for so long.

I am sorry for having kept you waiting so long. It is fine but can be a bit better:

I am sorry for having kept you waiting for so long.

I'm sorry having kept you waiting so long. Not so good. Better:

I'm sorry for having kept you waiting for so long.

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