Identify the independent and dependent variables in the following descriptions of experiments:

a. Students watched a cartoon either alone or with others and then rated how funny they found the cartoon to be.

b. A comprehension test was given to students after they had studied textbook material either in silence or with the television turned on.

c. Some elementary school teachers were told that a child's parents were college graduates, and other teachers were told that the child's parents had not finished high school; they then rated the child's academic potential.

d. Workers at a company were assigned to one of two conditions: One group completed a stress management training program; another group of workers did not participate in the training. The number of sick days taken by these workers was examined for the two subsequent months.