Controls are need in a case-control study because:
a. They are matched to the cases for the suspected etiolgoci factor under study.
b. They may be followed to determine if they develop the disease in question.
c. The increase the sample size so that statistical signifcance may be acieved.
d. The allow evalution of whether or not the frequency of exposure among cases is different from that among comparable person without the disease.
e. They do not have the exposure under study and thus allow evaluation fo whether the frequency of the disease differs among those who are or are not exposed.