Colin wants to find out how using fins increases swimming speed. If an independent groups design is used, the researcher would manipulate the variable by:
a. randomly requiring half the swimmers to swim without fins and the other half to swim with fins.
b. having all swimmers first swim without fins and then with fins.
c. having half the swimmers first swim without fins and then with fins and having the other half swim first with fins and then without fins.
d. requiring all swimmers to first swim with fins and then without fins.