In an immunodiffusion test, a strip of filter paper containing diphtheria antitoxin is placed on a solid culture medium. Then bacteria are streaked perpendicular to the filter paper. If the bacteria are toxigenic,
a. the filter paper will turn red.
b. a line of antigen–antibody precipitate will form.
c. the cells will lyse.
d. the cells will fluoresce.
e. none of the above