B-树的性质（续〉 是应该还是一定？ 4.All leaves have the same depth,which is the tree's height h. 5.Nodes have lower and upper bounds on the number of keys they can contain. We express these bounds in tms of a fixed integer t >2 called the minimum degree of the B-tree: a.Every node other than the 规定节点存储元素数量 keys.Every internal node other than the root t 的上下界，有何用途？ the tree is nonempty, the root must have at least one key. b.Every node may contain at most 2t-1 keys.Therefore,an internal node may have at most 2t children.We say that a node is full if it contains exactly 2t-1 keys.2B-树的性质（续） 是应该还是一定？ 规定节点存储元素数量 的上下界，有何用途？