Use Your Knowledge of the Parts of an Essay Question

Use your knowledge of the parts of an essay question (topic, key word, and limiting word) to answer questions bellow.
-In the question, "Discuss the reasons why, although tropical plants have very large leaves and most desert plants have very small leaves, cactus grows equally well in both habitats," the topic is

A) tropical plants.
B) desert plants.
C) cactus.
D) habitats.