Please answer the following question: Jack was so excited about getting a new backpack because his old one had a broken zipper and a hole in the bottom. Jack's elder brother took him to buy a new backpack. There were so many cool ones. How would he ever decide? He saw blue ones--his favorite color. He saw ones with his favorite action hero on the front. And then Jack saw a backpack with a black square design. When he opened it, he saw it had a hidden, secret small bag. He liked it, which made it different from others. "This is the one," he said, then carried it to the check-out counter. That evening, Jack carefully filled his new backpack with his school things. He put his special pen in the secret small bag. The next day in English class, Mr. Mack said, "Let's write an essay about earthquakes!" Jack looked for his pen in his backpack, but he couldn't find it! He through his desk and his pockets, but no pen. Jack asked if anyone had seen his pen, but no one had. Mr. Mack lent him a pencil, but Jack was feeling anxious because he lost his pen. When he got home, Jack told his brother he couldn't find his favorite pen. His brother said, "You must have put it in that secret small bag." Jack laughed and said, "That small bag was so secret that I forgot all about it." This text mainly tells us _ . A) a story of Jack's old backpack B) that Jack was studying earthquakes C) that Jack was old enough to do shopping D) a story of Jack's pen and new backpack
A:
D