In Pasteur's experiments on spontaneous generation, which of the following was kept constant throughout the experiment?

• A. The type of nutrient broth used in the flasks
• B. The exposure of broth to air in all flasks
• C. The shape of the flask openings
• D. The presence of microorganisms in the broth before boiling

Answer: (A)

Verified step by step guidance

1. Understand the purpose of Pasteur's experiment, which was to test whether microorganisms arise spontaneously or from existing microbes.
2. Identify the variables in the experiment: nutrient broth type, exposure to air, flask shape, and presence of microorganisms before boiling.
3. Recall that Pasteur used nutrient broth to support microbial growth, and this broth type was kept the same in all flasks to ensure consistency in the growth medium.
4. Note that the exposure to air was controlled by using swan-neck flasks, which allowed air in but prevented microbial contamination, so this was not identical in all flasks but carefully controlled.
5. Recognize that the presence of microorganisms before boiling was eliminated by sterilization, and the shape of the flask openings varied (swan-neck vs. straight), so these were not constant.