A、Associate what we see with words. B、Find special ways to remember things. C、Go picnic to release our tension. D、Use our memory

Memory, I think, is almost as important as air, water or sunshine to us. Without it, life can not go on. We wouldn’t be able to do anything since completing any task, however simple it might be, requires us to remember certain procedures, nor would we be able to communicate with people as we wouldn’t be able to remember the words to express our ideas or feelings.
    For us students, I think memory is particularly important. If we lose the faculty of remembering, we won’t be able to learn anything since learning unavoidably involves the use of memory. And if we cannot remember the knowledge we have acquired, what’s the point of learning it in the first place?
    Although everyone is born with the capacity to remember things, some of us have better memories than others.There are people who are said to have a photographic memory and can remember anything they have seen only once. However,those of us who have a poor memory needn’t panic, for there are ways to improve our memories. Paying attention, associating facts with images, visualizing images, and relaxing are some of the ways that I find especially effective. But the greatest secret to keep our memory sharp is to use it as frequently as possible.
19. What’s the speaker mainly talking about?
20. What can people with a photographic memory do?
21. What does the speaker recommend strongly in the end?