To address the problem of disproving spontaneous generation, which of the following options provides a key difference between the experiments of John Needham and Lazzaro Spallanzani?

• A. Needham boiled the broth to kill any existing microorganisms, whereas Lazzaro Spallanzani did not boil the broth.
• B. Spallanzani also boiled broth, but he boiled it for a shorter period than Needham.
• C. Spallanzani also boiled broth, but he did not seal the flask.
• D. Spallanzani boiled broth for a longer period and sealed the flasks more effectively, preventing contamination from the air.

Answer: (D)