In the Experiments Constructed by Pasteur to Disprove Spontaneous Generation

In the experiments constructed by Pasteur to disprove spontaneous generation, swan-necked flasks were used.Why was this shape of flask used in this experiment?

A) The shape of the glass neck allowed the bacteria into the flask and then into the media, but air could not enter.
B) The glass necks needed to be open to the air, yet constructed so that bacteria would settle in the lowest part of the neck.
C) These flask shapes were the easiest and cheapest to produce.
D) Because the glass necks were stretched out, the heat used to sterilize the medium inside of the flask could not kill the bacteria in the neck.