Four flasks are prepared with a broth that supports microbial growth. Broth A is boiled and then sealed to prevent the entry of air. Broth B is boiled and left open to the air. Broth C is boiled and left open, but the flask is fitted with an S-shaped neck. Broth D is boiled, left open to the air, and fitted with an S-shaped neck, but after two days the neck is removed. If the broths are examined after four days, which ones will contain living microorganisms?