At First, You Can Barely Concentrate in Your Psychology Class

At first, you can barely concentrate in your psychology class because the guy sitting behind you is wolfing down a hamburger smothered in onions for his lunch. After a minute, you no longer notice the smell of his sandwich because:

A) the airborne molecules released by the onions have blocked your odor receptor cells.
B) your thalamus is no longer processing olfactory signals.
C) your odor receptor cells have increased in sensitivity.
D) sensory adaptation has taken place.