Ross Times, the student newspaper of Ross College, printed a "What do you think?" column feature asking: "Do you think that the college is doing enough to provide student parking?" Anyone could mail in a response or use the paper's Web site to respond. In all, 126 answers were received. This is an example of:

(a) a voluntary response sample.

(b) a multistage sample.

(c) a simple random sample.

(d) a convenience sample.

(e) the placebo effect.