大脑是人体生命活动的“指挥部”，构成大脑的主要组织是（    ）

   Bob was cutting a branch（树枝） off a tree in his garden. While he was sawing（锯）, another man passed in the street. He stopped and said, “ Excuse me, but if you continue (继续)to saw（锯） that branch like that, you will fall down with it.” He said this because Bob was sitting on the branch and cutting it at a place between himself and the trunk（树干） of the tree.
   Bob said nothing. He thought, “ This is some foolish（愚笨的） person who has no work to do and goes about telling other people what to do and what not to do.”
   The man continued on his way. Of course, after a few minutes. The branch fell and Bob fell with it.
   “My God!” he cried. “That man knows the future!” and he ran after him to ask how long he was going to live. But the man had gone.