阅读下列短文，从短文后各题所给的A、B、C、D四个选项中，选出可以填入空白处的最佳选项。
Long ago, there lived a man in a village. The man was very rich, but he always 1 about his son David. David never thought about his future as he believed that his father had so much 2 that he could live a good life without doing any hard work.
The man wanted to teach his son how 3 hard work was. One day, he told his family that David must 4 his food every day, or he would stay hungry. The family didn't like the man's idea. So they gave David money secretly.
Every day, David gave the money to his father to get food. Then his father asked him to throw the money into the 5 behind their house. David didn't know why his father ordered him to do so 6 he still threw the money into the river without hesitation（犹豫）.
Some days went by, and the man 7 that his family had helped David. So, he told his family that he wanted to teach David a lesson. After hearing the man, the family all 8 not to help David.
To make money for his food, David had to 9 . Then he gave the money he earned（赚得） to his father and asked for food. Again, his father asked him to throw the money into the river. But this time, David refused to do so and said, "Father, I worked hard to make the money. I can't throw it into the river."
Hearing this, the man knew that his son had learned an important 10 .
