In a large city school system with $33$ elementary schools, the school board is considering the adoption of a new policy that would require elementary students to pass a test in order to be promoted to the next grade. The PTA wants to find out whether parents agree with this plan. Listed below are some of the ideas proposed for gathering data. For each, indicate what kind of sampling strategy is involved and what $($if any$)$ biases might result. Assume the schools are homogeneous and differ from each other.

a$)$ Mail a survey to the home of every student, and ask parents to fill it out and return it within a week.

Which kind of sampling strategy is involved?

A$.$ This sampling strategy is stratified sampling.

B$.$ This sampling strategy is a voluntary response sample.

C$.$ This sampling strategy is cluster sampling.

D$.$ This sampling strategy is an attempt at a census.