Factor A, B, or C can each individually cause a certain disease without the other two factors, but only when followed by exposure to factor X. Exposure to factor X alone is not followed by the disease, but the disease never occurs in the absence of exposure to factor X.
Factor X is:__________
a. A necessary and sufficient cause
b. A necessary, but not sufficient, cause
c. A sufficient, but not necessary, cause
d. Neither necessary nor sufficient
e. None of the above