In Which of the Following Examples Is the Independent Variable

In which of the following examples is the independent variable manipulated using the type technique?

A) A researcher is interested in the effects of rehearsal on memory performance. She has one group of students use rehearsal whereas a control group does not rehearse.
B) A researcher is interested in the effects of rehearsal on memory performance. She has one group of students rehearse items for 5 seconds, a second group for 10 seconds, and a third group for 15 seconds.
C) A researcher is interested in the effects of rehearsal on memory performance. She has one group of students rehearse each item for 10 seconds. She has another group rehearse each item for 20 seconds. Finally, the third group rehearses each item for 30 seconds.
D) A researcher is interested in the effects of rehearsal on memory performance. She has one group of students rehearse items individually, one item at a time. A second group rehearses using a grouping technique, in groups of five. The third comparison group rehearses "freely" as they see fit.