We are running Dijkstra's algorithm on this graph with source vertex $0.$ In the first round of the algorithm, we relaxed all outgoing edges of vertex $0.$ In the second round of the algorithm we relaxed all outgoing edges of vertex $1.$ Which vertex should the algorithm relax all outgoing edges of in the third round of the algorithm? Answer set:

a$.2$

b$.3$

c$.4$

d$.5$