Which observation by Alexander Fleming provided the crucial clue that led to the discovery of penicillin?

• A. A mold colony on a bacterial culture plate was surrounded by a clear zone where bacteria failed to grow
• B. A heated culture had fewer bacteria after incubation, indicating heat-labile substances kill microbes
• C. Patient sera from people recovering from infection always killed bacteria in vitro, implying antibodies were antibiotics
• D. All bacterial isolates from hospital patients were found to be naturally resistant to mold-derived compounds

Answer: (A)