How does positive cooperativity contribute to the sigmoidal shape of hemoglobin's oxygen binding curve?

It has no effect on the shape of the oxygen binding curve.

It causes hemoglobin to bind oxygen less efficiently, resulting in a hyperbolic curve.

It allows hemoglobin to bind oxygen more efficiently after the first molecule is bound, creating an S-shaped curve.

It prevents hemoglobin from binding oxygen, resulting in a linear curve.