You and I are friends, and each exchanges an apple for each other. After the exchange, there is still an apple each; if you have one kind of thought, I also have one kind of thought, and friends exchange ideas, then each of us has two kinds of thoughts. What is the main point supported by this passage? ['It is important for people to exchange ideas.', 'Friends need to exchange ideas more.', 'Thought exchange between friends is more valuable.', 'Exchange of ideas can improve people.']