阅读理解：阅读下列短文，回答文后问题，选择最佳答案

Robert had just moved into the street and he felt strange that he was not wanted. He knew that perhaps the other boys were trying to get an idea of what kind of a boy he was. This did not help him to make himself less lonely. He was new and he had to be tested. Still, proving himself would not be all that easy. He did not want to run with the boys or get into something against the law to prove that he was strong. No! He must show what he was made of in a more helpful way. That’s when he got the idea.

The next day was Saturday. He knew that most of the boys would be down on the playground and choose up sides for the Saturday game. Robert knew he could play well and that just might be enough to prove he was strong, and to make friends with them. He arrived early and did his step exercises. He shot the ball several times and did some other exercises—the most difficult and most wonderful in basketball. Then the boys came. Robert went through what he had done before the game and showed what he could do. No one said a word. The boys just looked at each other and thought about it. In the end, when it was all over, the biggest of the group just smiled and shook his head. Robert knew he had made it.

4. What did the biggest of the group mean by shaking his head?