Please answer the following question: Read the article and select the best answer. Article: There was once a farmer who lived near a road. It was not a busy road,but from time to time,cars passed the farm. Near the farm gate, there was a large hole in the road. this hole was always full of water, and the drivers of the cars could not see how deep the hole was. They thought it was probably not deep.So when they drove into the hole, they could not drive out because it was so deep. The farmer did not spend much time working on his farm. He spent most time watching the hole. Whenever a car drove into it, he would pull the car out with his tractor and he would charge the drivers for much more money than they had expected. One day, a driver of a car said to him, "You must have made a lot of money pulling cars out of this hole night and day." "Oh no," the farmer said, "I don't pull cars out of the hole at night. At night I fill the hole with water." Question: Why did the farmer fill the hole with water at night? Options: A: He was busy working on the farm during the day. B: He spent much time watching the hole by day. C: Some cars had driven into the hole at night. D: Nobody would be able to find the secret out.
Answer:
D