In order to obtain a sample of undergraduate students in the United States, a simple random sample of 10 states is selected. From each of the selected states, 10 colleges or universities are chosen at random. Finally, from each of these 100 colleges or universities, a simple random sample of 20 undergraduate students is selected. Thus, the final sample consists of 2,000 undergraduates.

This is an example of:

(a) simple random sampling.
(b) stratified sampling.
(c) multistage sampling.
(d) convenience sampling.
(e) none of the above.