Question

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In this excerpt from "The Road Not Taken" by Robert Frost, which pair of lines casts doubt on the certainty the speaker felt when choosing one path over the other?
Two roads diverged in a yellow wood,
And sorry I could not travel both
And be one traveler, long I stood
And looked down one as far as I could
To where it bent in the undergrowth;

Then took the other, as just as fair,
And having perhaps the better claim,
Because it was grassy and wanted wear;
Though as for that the passing there
Had worn them really about the same,

And both that morning equally lay
In leaves no step had trodden black.
Oh, I kept the first for another day!
Yet knowing how way leads on to way,
I doubted if I should ever come back.

Answer 1

The answer is
Yet knowing how way leads to on to way,
I doubted if I should ever come back.

Answer 2

I doubted if I should ever come back.