A small math department has six faculty members and 30 students. It can send 6 people to the national convention and they would like to send 4 students and 2 faculty members. Of the 30 students, 4 are chosen randomly, and then 2 faculty members are randomly chosen from the six. This is an example of
A. simple random sampling.
B. stratified random sampling.
C. voluntary response sampling.
D. a census.