Scientists Are Always on the Hunt for New and More

Scientists are always on the hunt for new and more effective antibiotics. Suppose that a new antibiotic (antibiotic X) is discovered that may be able to kill MRSA (methicillin-resistant S. aureus) , a particularly dangerous and resistant strain of bacteria. Which experiment would be best to conduct to determine if antibiotic X is more effective at killing MRSA than the antibiotic penicillin?

A) Treat a sample of MRSA with both antibiotic X and penicillin, and count how many MRSA cells are alive after three days.
B) Treat a sample of MRSA with only antibiotic X and a second sample of MRSA with only penicillin, and count how many MRSA cells are alive in each sample after three days.
C) Treat a sample of MRSA with only antibiotic X, and count how many MRSA cells are alive after three days.
D) Treat a sample of MRSA with only penicillin, and count how many MRSA cells are alive after three days.