Which of Mill's methods is involved in the In a study of the effects of dental health on heart ...

Which of Mill's methods is involved in the In a study of the effects of dental health on heart disease, investigators found that compared with people who had all their teeth; those with 1-6 missing teeth (not counting those lost to injury or orthodontics) were 1.2 times as likely to have cardiovascular disease. Those with 7-31 missing teeth were 1.64 times as likely, while those with no teeth were 1.87 times as likely.
 
  a. Method of Concomitant Variation
  b. Method of Difference
  c. Method of Residues
  d. Method of Agreement