Which of the following is an example of variable-ratio reinforcement?

a. College acceptance letters arrive around the date of April 1.

b. Percy gives his dog a cookie whenever his dog walks by strangers without barking.

c. Esmeralda disliked the substitute teacher, so she scowled every time she looked at him.

d. Judy discovered a shark’s tooth after several hours of searching for one on the beach.

e. When Stu had been working on his homework for one hour, his mother allowed him to go outside and play.