6. Dr. Vaz conducted an experiment in which she randomly assigned her participants to one of two conditions. In the first condition, the participants were shown visual images of common objects and then 1 hour later asked to recall as many of the objects as they could remember. In the second condition, the participants heard the names of the same objects and then 1 hour later were asked to recall as many of the objects as they could. Dr. Vaz then compared the number of items recalled for these two groups of participants. In this experiment, the independent variable is .

a. the number of items recalled

b. whether the participants saw or heard the objects

c. the sex of the participants

d. the room in which the participants were tested