Question

multiple choice question pasteurs experiment with sterile broth in swan - necked flasks demonstrated that air and dust were the source of microbes because microbes grew only
○ when the neck was broken off and dust could fall directly into the flask
○ in the flasks that had been sterilized by boiling
○ in the flasks with the swan - necked shapes

Explanation:

Pasteur's swan - necked flask experiment showed that when the swan - neck was intact, dust and microbes in the air were trapped in the bend and no growth occurred in the broth. Only when the neck was broken off and dust could directly fall into the flask did microbes grow, indicating air and dust as the source of microbes.

Answer:

when the neck was broken off and dust could fall directly into the flask