A child with otitis media has had three ear infections in the past year. The child has just ...

A child with otitis media has had three ear infections in the past year. The child has just completed a 10-day course of amoxicillin (Amoxil) with no improvement.
 
  The parent asks the nurse why this drug is not working, because it has worked in the past. What will the nurse tell the patient?
  a. Amoxicillin is too narrow in spectrum.
  b. The bacteria have developed a three-layer cell envelope.
  c. The bacteria have developed penicillin-binding proteins (PBPs) that have a low affinity for penicillins.
  d. The bacteria have synthesized penicillinase.