Staph infections, caused by the bacterium Staphylococcus aureus, are increasing in frequency and are more difficult to treat with antibiotics. Many staph infections are now caused by MRSA, and methicillin-resistant strain of S. aureus. What explains the more widespread phenomenon of MRSA prevalence in staph infections?

A. Vaccines became less effective because of the changes in the human immune system

B. Antibiotics caused mutations in the bacteria

C. hospitals decreased their sanitation practices

D. Resistant bacteria already existed from random mutations and the excessive use of antibiotics increased the frequency of the resistant strain