Factors A, B, or C can each individually cause a certain disease with or without the other two factors being present, but only when followed by exposure to factor X. Exposure to Factor X alone is not followed by disease, but the disease never occurs in the absence of exposure to factor X. Factor X is:

Select one:

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