If you do not use your arms or your legs for some time, they become weak. When you start using them again, they slowly become strong again. Everybody knows this. Yet there are many people who do not seem to know that one's memory works in the same way. When someone says that he has a good memory, he really means that he keeps his memory in practice by exercising it regularly. When someone else says that his memory is poor, he really means that he does not give it enough chance to become strong. If a friend complains that his arms are weak, we know that it is his own fault. But if he tells us that he has a poor memory, many of us think that his parents are to blame, or that he is just unlucky, and few of us realize that it is really his own fault. Not all of us can become very strong or very clever, but all of us can improve our memory by the same means -- practice. Have you ever noticed that people who cannot read or write usually have a better memory than those who can? Why is this? Of course, because people who can't read or write have to remember things: they cannot write them down in a little notebook. They have to remember dates, prices, names, songs and stories, so their memory is exercised all the time. So if you want a good memory, learn from those who cannot read or write: practise remembering. According to the passage, few people know that   _  .
arms or legs become weak if they are not used for some time when they start using their arms or legs again, they slowly become strong again a person's memory becomes weak if he does not practise remembering things it is their own fault if their arms or legs are weak