In Pasteur's experiments on spontaneous generation, which setup served as the control to demonstrate that microbial growth did not occur without exposure to air?

• A. Flasks containing boiled broth but not exposed to air
• B. Flasks with swan-necked necks left unbroken and upright
• C. Flasks with straight necks left open to air
• D. Flasks sealed completely with corks

Answer: (B)

Verified step by step guidance

1. Understand the purpose of Pasteur's experiment: to test whether microbial growth occurs spontaneously or requires exposure to air containing microorganisms.
2. Identify the experimental setups: Pasteur used flasks with swan-necked necks (curved necks), flasks with straight necks open to air, flasks sealed completely with corks, and flasks containing boiled broth but not exposed to air.
3. Recognize that the control setup should demonstrate that no microbial growth occurs when air is present but microbes are prevented from reaching the broth.
4. Recall that the swan-necked flasks allowed air to enter but trapped dust and microbes in the curve, preventing them from reaching the broth, thus serving as the control to show no growth without microbial contamination.
5. Conclude that the flasks with swan-necked necks left unbroken and upright served as the control demonstrating that microbial growth did not occur without exposure to airborne microbes.