Which statement best critiques the structure of this selection?
a) The selection is unified because it focuses on a single subject, the Sahara.
b) The selection is unified because it contrasts two related regions, the Sahara and the Sahel.
c) The selection lacks unity because it mentions Antarctica, which is "very different" from the main topic of the Sahara.
d) The selection lacks unity because it begins by contrasting the Sahara and the Sahel and ends by contrasting camels and horses.