Narrator Listen to a lecture talk about bass in business class. What is the main topic about this lecture?

[Professor]
    Today, we’ll discuss the invaluable things .eh.. It’s hard to distinguish the tree treasure. Now, I will tell you a story: When an American teenager named Joshua wanted a bass, his father went out and bought him one. But, much to Joshua’s dismay, his dad gave him an old, ugly one instead of the new one he wanted. For years, Joshua carried it around with resentment, always vowing to buy a better instrument as soon as he could afford to. Then one day Joshua took the bass in for a minor repair. While in the shop, he was shocked to learn that his bass was a valuable antique. According to experts, the one-hundred-and-fifty-year-old bass was one of only twelve ever made, thereby making an under appreciated gift a truly priceless treasure.
    While Joshua’s good fortune came unexpectedly, others try to make their own luck: Whenever they can, they search the dusty comers of antique stores for overlooked treasures. But what, exactly, are they hoping to find? What is it that makes one old thing valuable, while other old things are just--old?
    Joshua’s break was particularly lucky because, despite his neglect for his instrument, his old bass wasn’t badly scratched or damaged. Generally, the closer to its original condition something is, the greater its value will be. But the quality of an item also affects price: regardless of how old an object is, if it is poorly-made, it won’t be worth any more than something made yesterday. Of course, one-of-a-kind items like Joshua’s guitar are valuable because they are so rare. But sometimes, consumer demand can raise the price of even very common things, such as mass-produced toys. A surge in demand drives up an object’s market value and can lead to the latest collectible being snapped up by consumers.
    Consider a pair of 1986 Air Jordans. Even a used pair can fetch a price of $10,000 in some Tokyo shops. The fact that they are unattractive and mass-produced doesn’t take away from their value, simply because Japanese teenagers think they are fashionable.
    Because one man’s trash is another man’s treasure, people often don’t realize that they have a valuable object on their hands. Take baseball cards for example: memorabilia shops have shelves lined with old cards, many selling for hundreds of dollars. But where do you think these cards come from?
    The answer is from the attics and storage closets of people who traded them in without being aware of their value, of course. If these unlucky fools hadn’t parted with their childhood treasures, savvy sports fans would not be able to pick them up for a good price at their local store.
    On the other side of the coin are antique hunters who think they are buying priceless treasure only to later discover that they’ve bought a piece of worthless junk. The best way to ensure you aren’t disappointed is to only purchase things you love. Even if they aren’t expensive or rare, certain objects fill us with nostalgia--and it is this potential to remind us of days gone by that makes them true treasures.
17. What is the main topic about this lecture?
18. Why does the speaker mention "Joshua got an old bass from his father"?
19. Why did Joshua take his guitar to the shop?
20.  Which choice based on the statements "a surge in demand drives up an object’s market value and...being snapped up by consumers" is NOT correct?
21. Which of the following statements does the professor support?
    Narrator listen again to a part of the lecture, then answer the question.
[professor]   On the other side of the coin are antique hunters who think they are buying priceless treasure only to later discover that they’ve bought a piece of worthless junk. The best way to ensure you aren’t disappointed is to only purchase things you love. Even if they aren’t expensive or rare, certain objects fill us with nostalgia--and it is this potential to remind us of days gone by that makes them true treasures.
22. Why does the professor say thisΩ?
    "The best way to ensure you aren’t disappointed is to only purchase things you love. Even if they aren’t expensive or rare, certain objects fill us with nostalgia--and it is this potential to remind us of days gone by that makes them true treasures."