B-树的性质(续) 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 terms of a fixed integer t> 2 called the minimum degree of the B-tree a. Every node other than the root must have at least t-l keys. every internal node other than the root thus has at least t children. If the tree is nonempty the root must have at least one key b. Every node may contain at most 2t-I keys. Therefore, an internal node may have at most 2t children. We say that a node is full if it contains exactly 2t-l keysB-树的性质（续）