Which of these best describes why certain bacteria have become resistant to penicillin? a) the penicillin in the 1970's was much weaker that the penicillin used today, thus it was not as effective. b) penicillin was not effective in treating any bacterial diseases, so the doctors had to find new treatments. c) these new bacterial strains came out of the jungles and forests already equipped to be penicillin-resistant. d) penicillin was so heavily prescribed by doctors that it led to certain, more fit, strains of bacteria to be able to survive.