If you do not use your arms or your legs some time, they become weak; when you start using them again, they slowly become strong again. Everybody knows this and nobody would think of questioning this fact. Yet there are many people who do not seem to know that 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 frequently either consciously  or unconsciously. When someone else says that his memory is poor, he really means that he doesn't give it enough chances to become strong.  If a friend complains that his arms and legs 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 his parents are to blame and that he is just unlucky, and few of us realize that it is just as much as his own fault as if it is his arms or legs that were weak. When someone has a good memory, this is because   _   .
his parents are very clever he was born clever he keeps using his memory very often he always does things in a correct way