Read the following passage and mark the letter A, B, C or D to indicate the correct answer to each of the questions from 21 to 25.

Once upon a time, there was a man with a fine house, healthy animals and fertile land. He was also blessed with a handsome, loving son. But the man was worried about his son, because the boy had grown up knowing only good times and good fortune.

"What will happen when something goes wrong for him?" wondered his father. "How will he know what to do?"

So one day, the man asked the boy to go into the forest and collect wood. To help him carry the wood home, the man gave the boy an old cart and two horses. As he was leaving, the man said to his son, "If the cart breaks down while you are in the forest, don’t worry. Necessity will teach you what to do."

The boy collected as much wood as he could load onto the cart. But when he turned to come home, the cart broke under the weight of the wood. "I hope that Father’s friend lives nearby," thought the boy, and he started crying out, "Necessity! NE-CESS-ITY! Where are you?" The boy called and called, but no one came, so he decided to fix the cart himself.

When he arrived home, he said to his father," You said Necessity would teach me to fix the cart, but I couldn’t find him anywhere. I had to fix it all by myself."

The man smiled to himself and thought, "Necessity really is the best teacher."

Why was the boy’s father worried?

If energy ________ inexpensive and unlimited, many things in the world would be different.