Which of the following is a sentence fragment?
a. The chain of events unfolded this way: Clara and Melvin went fishing, their boat engine broke down, and a storm rolled in.
b. The slogan—"Everyone needs one"—for the product.
c. High above the city a balloon floated it drifted for hours on end.
d. The writer furrowed his brow, glared at the paper, and, eventually, wrote.