In one experiment the professor put a monkey in a mom where there were several small boxes. Some boxes were inside other boxes. One small box had some food inside of it. The professor wanted to watch the monkey and to find out how long it would take the monkey to find the food. The professor left the room. He waited a few minutes outside the door. Then he knelt down and put his eye to the keyhole. What did he see？ To his surprise he found himself looking directly in to the eye of the monkey. The monkey was looking at the professor through the other side of the door.
What was the purpose of the professor's experiments？

A. To find out how clever monkeys were.
B. To test the intelligence of different animals.
C. To compare the difference between man and the monkey.
D. To find out how monkeys search for food.