To gather information about the validity of a new standardized test for tenth-grade students in a particular state, a random sample of 15 high schools was selected from the state. The new test was administered to every 10th-grade student in the selected high schools. What kind of sample is this?

(a) A simple random sample

(b) A stratified random sample

(c) A cluster sample

(d) A systematic random sample

(e) A voluntary response sample