challenge: What is a key difference between Wautersiella falsenii and Empedobacter brevis?
  
  A. Wautersiella falsenii is motile, while Empedobacter brevis is non-motile
  B. Wautersiella falsenii is urease-positive, while Empedobacter brevis is urease-negative
  C. Wautersiella falsenii is lactose fermenting, while Empedobacter brevis is non-lactose fermenting
  D. Wautersiella falsenii is Gram-positive, while Empedobacter brevis is Gram-negative
  Answer: 