Which of Mill's methods is involved in the following? Professor Ames, who enjoys wine, finds that ...

Which of Mill's methods is involved in the following? Professor Ames, who enjoys wine, finds that he often awakes very early on the morning following an evening of wine consumption, but on other such mornings he does not. Keeping track of the wines consumed, he finds that early waking following the drinking of cabernet sauvignon (a red wine), zinfandel (also red) and pinot noir (red), but not after drinking Chablis (a white wine), chinin blanc (white) or Riesling (white).
 
  a. Method of Difference
  b. Method of Agreement
  c. Joint Method
  d. Method of Concomitant Variation