A study was performed to compare the effects of three ointments (labeled A, B and C) for treating eye infections in dogs. A trained veterinarian examined each dog and classified its eye infection as either minimal or severe. Within each of the two infection classes, the dogs were randomly assigned to receive ointment A, B or C. After a week of treatment, the eyes were checked to see the condition of the infection.
What type of design was used?

a. Stratified random sample
b. Cluster sampling
c. Randomized Blocking design
d. Multistage sampling
e. Simple random sample