A、To show that enzymes are very effective. B、To point out that enzymes can sometimes fail to work. C、To explain what enzymes are

Let’s begin today by discussing enzymes. Enzymes are what make many of the body’s biochemical reactions possible. Actually biochemical reactions can take place without them, but at much lower rates. In fact an enzyme may cause a reaction to proceed billions of times faster than it would otherwise. Before I go on to the biochemical specifics of how this works, let me provide a figurative example. I think it will help illustrate the power of enzymes more clearly. Now, suppose you got a bag and you put a bunch of locks in it, just small padlocks. Then you put in all the keys that go with the locks. And you closed the bag and shook it hard. No matter how long you shook, chances are very small that any key would get inserted in any of the locks. But if you took them all out of the bag and this time used your hands to insert the keys in the locks, you could combine them much quicker.
    Enzymes act like your hands, quickly allowing chemical reactions that would otherwise take much longer. Now, there are two reasons that enzymes are so effective at enabling biochemical reactions. First, enzymes greatly reduce the amount of energy required to start the reactions, and with less energy needed the reactions can proceed a lot faster than they could without the enzyme. The second reason is that a small amount of an enzyme is needed to enable the biochemical reaction. That’s because the chemical structure of the enzyme itself does not become altered as if enables the reaction. So a single enzyme can be used to start the same biochemical reaction over and over again.
Questions 21 - 25 are based on the passage you have just heard.
21. What is the passage mainly about?
22. What point does the professor make about an enzyme when it is involved in a biochemical reaction?
23. What do enzymes do to help start a biochemical reaction?
24. Why does the professor talk about locks and keys?
25. What is enzymes?