Explain what adjustments if any need to be made in Dijkstra's algorithm and/or in an underlying graph to solve the following problems.
a. Solve the single-source shortest-paths problem for directed weighted graphs.
b. Find a shortest path between two given vertices of a weighted graph or digraph. (This variation is called the single-pair shortest-path problem.)
c. Find the shortest paths to a given vertex from each other vertex of a weighted graph or digraph. (This variation is called the single-destination shortest-paths problem.)
d. Solve the single-source shortest-paths problem in a graph with nonnegative numbers assigned to its vertices (and the length of a path defined as the sum of the vertex numbers on the path).