The pizza delivery guy opens the door to the classroom and students in the front row smell the pizza after about 1 minute. Students in the middle of the room smell it after about 2 minutes. Students in the back of the room smell it after about 3 minutes. The MECHANISM OF TRANSPORT of the pizza odorant molecules from the source (the pizza at the door) to distant parts of the room is
A. bulk flow
B. carrier-mediated transport
C. dispersion
D. diffusion
E. co-transport