Would you use the adjacency list structure or the adjacency matrix structure in each of the following cases? Justify your choice.

a$)$ The graph has $10,000$ vertices and $20,000,000$ edges, and it is important to use as little space as possible.

b$)$ You need to answer the query areAdjacent as fast as possible, no matter how much space you use. areAdjacent is an operation that tests if $2$ vertices are adjacent.