Suppose that a philosophy professor is able to monitor the amount of time her logic students spend ...

Suppose that a philosophy professor is able to monitor the amount of time her logic students spend working on a computerized tutorial program. When the students spend 5 hours on the program, their average grade increases by 4 percentage points. When they spend 10 hours on the program, their average grade increases by an additional 4 percentage points, and when they spend 15 hours working on the program, their grades increase by yet an additional 4 percentage points. The professor concludes that work on the tutorial program causes an increase in student grades. What method did the professor use in drawing this conclusion?
 
  A) The method of difference.
  B) The method of agreement.
  C) The method of concomitant variation.
  D) The method of residues.
  E) The joint method of agreement and difference.