Which of the Following Scenarios Best Represents Variable-Ratio Schedule

Which of the following scenarios best represents variable-ratio schedule?

A) Umar gives his dog a treat every time the dog barks on his command.
B) Agatha frequently studies the material from her physics class because she knows that her professor will conduct a pop quiz once every week.
C) Doreen does not study for her accounting exams until one or two nights before the test is scheduled to take place.
D) Rishi continues to insert money into slot machines because he never knows how many pulls of the arm it will take to win the jackpot.