one thing left to do / one thing left

Hello.

1a. There is only one thing left to do: apologize while you have the chance.
1b. There is only one thing left: apologize while you have the chance. (I made this sentence.)

When I do a search on the Internet, I find there are two versions about 'one thing left' like the above. I think 'to do' in 1a is unnecessary. Threfore, both 1a and 1b are all possible and grammatically correct. Am I right?

Thank you always~.

I think it is necessary. I would also write it "There is only one thing left to do: to apologize while you have the chance".

Andygc said:

I think it is necessary. I would also write it "There is only one thing left to do: to apologize while you have the chance".

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Can't that be written with the to do out?

There's only one thing left: to apologize while you have the chance.

Some might. I wouldn't.
There's only one thing left: a dried-up sandwich on a dirty plate.
The difference is that it is something to do (an action) that is left, not just a thing.