Check the picture below.
notice the first depiction there, the plane is parallel to the base, chopping off the cone.
The 9-pack is the least per bottle. This is because...
3.79/6 = $0.63 per 1 bottle (6 pack)
4.50/9 = $0.50 per 1 bottle (9 pack)
6.89/12 = $0.57 per 1 bottls (12 pack)