A bacterial culture (200 μL containing 1.8 × 109 cells) was treated with an antibiotic Z (50 μg per mL) for 4 h at 37°C. After this treatment, the culture was divided into two equal aliquots. Set A: 100 μL was plated on Luria agar. Set B: 100 μL was centrifuged, the cell pellet washed and plated on Luria agar. After incubating these two plates for 24 h at 37°C, Set A plate showed no colonies, whereas the Set B plate showed 0.9 × 109 cells. This experiment showed that the antibiotic Z is

Answer: Option A