In a within-subjects design, individual differences (participant variables) are a problem because a. they can become confounding variables.  b. they can increase the variability. c. they can become...

In a within-subjects design, individual differences (participant variables) are a problem because

a. they can become confounding variables.

 b. they can increase the variability.

c. they can become confounding variables and they can increase the variability.

d. individual differences are not a problem in a within-subjects design.