Professor Knotso Swift wants to know whether students learn better with or without an accompanying textbook for his lectures. What sampling technique (simple random, stratified random, cluster, convenience) is Professor Swift using in each of the following studies?

a. He uses the textbook for students in one of his classes and does not use the textbook for students in another of his classes.

b. He announces the intent of his study and lets students who state that they prefer to use the textbook use it and lets students who state that they prefer not to use the textbook not use it.

c. He classifies the students in each of his classes by class year, sex, and cumulative GPA. From each of these three groups, he selects students to take the course with the textbook and students to take the course without the textbook.

d. Starting at an arbitrary point in his class roster of names, he assigns every other student either to receive a textbook or not to receive a textbook.