Evaluate each of the following as a technique for obtaining a random sample of subjects.

a. An experimenter obtains subjects by asking every third driver stopping at the stoplight on the corner of Hollywood and Vine streets in Los Angeles to be in an experiment.

b. A researcher places an ad in a local pa- per asking for volunteers for a psychol- ogy experiment.

c. An experimenter calls every fourth number in the phone book and asks for volunteers for a research project.

d. A wealthy graduate student posts signs on the university bulletin boards offering $8 per hour for participating in a 2-hour perception experiment.