Ampicillin and other modern antibiotics kill a much wider variety of bacteria than penicillin does.
Conclusion (Thus)
Newer antibiotics are likely to result in an outbreak of diseases caused by drug-resistant bacteria.
Evidence
Newer antibiotics (like ampicillin) kill a much wider variety of bacteria than penicillin does, and since they make drug companies more money, we could see a situation in which there isn't as much penicillin available and doctors have to prescribe these more powerful antibiotics even when that would otherwise be unnecessary.
Evaluate
The blank is prefaced by a Support Indicator, "since" (fabs = for, after all, because, since), so we know we're trying to pick an answer that would support the previous claim.
Why would newer antibiotics, which kill a much wider variety of bacteria than penicillin make us more likely to have an outbreak of diseases caused by drug-resistant bacteria?
(If we have real-world knowledge about the fear of over-prescribing antibiotics, then we probably understand that the concern is that when you kill off all the bacteria that antibiotics CAN kill, you leave only the bacteria that antibiotics CAN'T kill to survive / thrive / replicate. Soon the dominant strains of bacteria are the ones that resist our drugs.)
Goal
If we don't already have that anticipated sense of where this is heading, then we just need to look for an idea that connects to the emergence of "drug-resistant bacteria".