Which Laboratory Example Reflects a Fixed-Ratio Schedule of Reinforcement

Which laboratory example reflects a fixed-ratio schedule of reinforcement?

A) A pigeon pecking a disk receives food every 6 minutes.
B) Every 7th time a rat presses a lever, it receives food pellets.
C) Every 6 minutes, a rat receives food for pressing a lever. Then the schedule changes to every 5 minutes, then to every 9 minutes.
D) A pigeon pecking a disk receives food after 4 pecks, then after 7, after 5, and then after 6.