Is the following an example of simple random, cluster, systematic or convenience sampling?
group the students in a local college by year and then randomly select 15 students from each group.

The following is an example of a cluster sampling.

Cluster sampling is a sampling technique used when "natural" but relatively heterogeneous groupings are evident in a statistical population. It is often used in marketing research. In this technique, the total population is divided into these groups (or clusters) and a simple randomsample of the groups is selected.

Were is the diagram?