"scientists will often grow bacteria in prepared petri dishes. in some experiments, the petri dish will also contain paper disks soaked in a particular antibiotic. the area where the bacteria do not grow is called the zone of inhibition, or zoi. the diameter of the zoi indicates the effectiveness of the antibiotic.
the zoi data collected by one scientist while trying to determine which disk (a, b, c, or d) is most effective at killing streptococcus bacteria are: disk d–9 mm, disk b–8 mm, disk c–0 mm.

determine the diameter in millimeters (mm) of the zoi for antibiotic disk a shown below"