Once a 10-year-old boy decided to learn judo   though he had lost his left arm in a car accident. The boy began lessons with an old Japanese sensei, a judo teacher. Three months later, he was still practising with the same move. So he asked, "Sensei, shouldn't I be learning more moves?" "This is the only move you know, but this is the only move you'll ever need to know," the sensei replied. The boy kept training. Months later, the sensei took him to his first match. Surprisingly, the boy easily won his first two matches. The third match was more difficult, but after some time, his opponent   became impatient. The boy used his one move to win the match and now was in the final. This time, his opponent was bigger, stronger, and more experienced. For a while, the boy seemed to lose the game. Being worried that the boy might get hurt, the referee   called a break. Just before the referee wanted to end the match, the sensei came to him. "No," the sensei said, "let him go on." Soon after the match began, his opponent made a big mistake: he dropped his guard  . Suddenly, the boy won the match. On the way home, the boy and the sensei talked about every move in each match. Then the boy gathered the courage to ask, "Sensei, how did I win the match with only one move?" "You won for two reasons," the sensei answered. "First, you've learned one of the most difficult throws in all of judo. Second, the only known guard for that move is for your opponent to catch your left arm." The boy could win all the matches mainly because   _  .
his opponents made more mistakes than he did his opponents were more impatient than he was the boy was better at guard than his opponents were the boy's biggest weakness became his biggest strength