A group of veterinarians at a major veterinary hospital was interested in investigating a possible link between enteroliths, stones that form in the colon of horses, and diet. They decided to conduct a survey of feeding practices of horses in the hospital's state. They created a survey questionnaire and decided to administer it to the owners of every fifth horse being treated at the hospital. The sample is a:

A. stratified random sample.
B. simple random sample.
C. volunteer sample.
D. convenience sample.