Doctors have fought infections with penicillin for over 50 years. recently, penicillin has not worked as well as before. which of the following explanations can account for this observation? a. some bacteria are resistant to the drug. the continued use of the drug has killed all of these bacteria. b. penicillin use has made all bacteria resistant to it. c. some bacteria are resistant to penicillin. the continued use of the drug has killed all but these bacteria. d. bacteria eat penicillin, so it is not effective anymore.