Use the excerpt from the poem below to answer the question.

What is the rhyme scheme of the following poem by Robert Frost?

He gives his harness bells a shake
To ask if there is some mistake.
The only other sound's the sweep
Of easy wind and downy flake.

The woods are lovely, dark and deep,
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.

a. AAAABBBB
b. AABABBBB
c. ABCDABCD
d. ABABABAB