Identify the sampling method: simple random sampling, systematic sampling, convenience sampling, or stratified sampling. a) A magazine predicts Grammy Award winners by polling readers who voluntarily respond by e-mail. b) A study of the use of antidepressants selects 50 participants between the ages of 20 and 29, 50 participants between the ages of 30 and 39, and 50 participants between the ages of 40 and 49. c) Student ID numbers are randomly selected by a computer for a survey of student opinions on college athletics. d) Every 100th cast iron pipe that comes off an assembly line is tested for its strength.