code: def chain_and_fork_graph():
      edge_list = [("A", "B"), ("B", "C"), ("B", "D"), ("D", "C")]
      G = nx.DiGraph(edge_list)
      return G
  
imports: from itertools import combinations
  import pytest
  import networkx
label_desc: Use image node_id 12 for calling a global function with example usage: chain_and_fork_graph() and returns: G