Consider the Following Experiment on Helping Disabled Persons

Consider the following experiment on helping disabled persons. One Monday, in a campus area with park benches where students often meet, an investigator placed one of two confederates: either a handicapped woman in a wheelchair or a nondisabled woman. The disabled woman was there between the hours of 9:00 a.m. and 12:00 noon, and the nondisabled woman was there between the hours of 1:00 and 4:00 p.m. While in the setting, every fifteen minutes each woman asked a randomly chosen passerby to help her find a lost earring. An observer, appearing to be reading a book while seated on a nearby bench, recorded the amount of time each subject spent helping to find the earring. If the subject had not left in fifteen minutes, the confederate pretended to find the earring.
a. What is the independent variable in this study?
b. What variable is confounded with the manipulation of the independent variable? (Identify a specific variable, not a class of extraneous variables such as history, maturation, etc.)