challenge: What is the definition of the graph of a function f:X→Y?
  
  A. The set of all x in X where f(x) is defined
  B. The set of all (x, y) in X×Y such that y=f(x)
  C. The set of all (x, y) in X×Y such that x=f(y)
  D. The set of all y in Y where f(x) is defined
  Answer: 