4. (a) (5%) According to the definitions of depth first number and low value, describe the sufficient and necessary condition of for a vertex u to be an articulation point. -velation ex u to be an articulation point (6) (10%) Consider a graph with 10 vertices shown as below. Assume that the graph is represented an the ative of will be clrot by adjacency lists and each adjacency list is sorted by vertex indices in an increasing order. If the start vertex is vertex 5, give the depth first numbers and low values of all vertices. 9 PILI231415161718191 35 od 0100013 (C) (5%) List all biconnected components and articulation points of the graph given in (b). 5. (10%) Consider the 2-way merge on disk. Assume there are 6000 records in disk to be sorted using 00 a computer with an internal memory capable of sorting at most 1000 records. Also assume that disk 1/0 is with block length of 250 records. Let to be the I/O time, including maximum seek time, maximum latency time, and transmission time, for a block of records. What is the total I/O time for the external sorting? 2 000)