Ampicillin and other modern antibiotics kill a much wider variety of bacteria than penicillin does. They also carry higher profit margins, so drug companies now have an incentive to stop manufacturing the older, less profitable antibiotics. This could cause a penicillin shortage, forcing doctors to use the much more powerful new antibiotics in cases where they might otherwise be unnecessary. Thus, these newer antibiotics are likely to result in an outbreak of diseases caused by drug-resistant bacteria, since __. The conclusion of the argument is most strongly supported if which one of the following completes the passage? ['drug-resistant bacteria flourish in the absence of competition from a wide variety of other bacteria', 'most bacteria that are resistant to penicillin are not resistant to ampicillin and other modern antibiotics', 'a shortage of penicillin would drive up its price and profit margin', 'treatment of diseases with the powerful new antibiotics is much more expensive than treatment with the older ones']