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?

And be one traveller,long as I could

And looked down one as far as I could

Yet knowing how way leads on to way,

I doubted if I should ever come back.

Explanation:

I am taking the same test. This is my answer and it seems that is should be the most reasonable pair of lines...

...which pair of lines casts doubt on the certainty he speaker felt when choosing one path over the other?

it could change in many ways and by that i mean income can go lower or higher depending on how hard you work. i hope this .

he was looking for nothing because this question had no complete sentence.