Final answer:
The sound device used in the first and third lines of "Birches" by Robert Frost is repetition.
Step-by-step explanation:
The sound device used in the first and third lines of Robert Frost's poem "Birches" is repetition. Repetition is the literary device where words, sounds, or phrases are used more than once for emphasis or effect. In the given lines, the phrase "bend to left and right" is repeated to create a rhythmic and melodic quality in the poem.
Learn more about sound device in poetry