Which of the following from "to build a fire"is an effective use of juxtaposition?

a. several times he stumbled, and finally he tottered, crumpled up, and fell.

b. the dog sat "in front of him, facing him, curiously eager and intent

c. the man drowsed off into "the most comfortable and satisfying sleep he had ever known.

d. … the man reached in his pocket for a second piece of birch-bark.