[B+ tree] (20) Consider the following B+ tree (partly shown) defined on attribute A with an integer domain in a relation r. Leaf nodes are linked (the links are not shown in the figure). The nodes are annotated with a label (e.g., "L1" for leaf node 1). Answer the following questions. (1) Consider the subtree B. Give the permissible ranges of the search keys in B's nodes. (2) Give the labels of all the nodes that need to be visited to answer the query: A=57 (r) (3) Answer the same question, but for the query: "find the records with search keys in [68,100) ". (4) Subtree C is not shown. What can we infer about the contents and shape of this subtree