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?

A. Two roads diverged in a yellow wood, And sorry I could not travel both

B. And be one traveler, long I stood And looked down one as far as I could

C. Though as for that the passing there Had worn them really about the same,

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