A. "The combined effect of both antibiotics is greater than each of them alone."
B. "The first antibiotic was not strong enough to work."
C. "We have not yet isolated the bacteria, so the two antibiotics are given to cover a wide range of microorganisms."
D. "We can give a reduced amount of each one if we give them together."
Aminoglycosides are often used in combination with other antibiotics, such as beta-lactams or vancomycin, in the treatment of various infections because the combined effect of the two antibiotics is greater than that of either drug alone.