For each of the following degree sequences, either draw a simple graph (simple digraph, respectively) with the degree sequence, or explain why such a simple graph does not exist.
a) Six vertices with degree sequence (4,3,2,2,2,1)
b) Five vertices with degree sequence (4,4,3,3,1)
c) Fours vertices with in-degree sequence (1,2,1,0)
and out-degree sequence (1,0,1,1)