If One Observatory Site Is Described as Having Better "Seeing

If one observatory site is described as having better "seeing" than another observatory site,what is it that is BETTER at the first site?

A) The sky is more transparent and there is less haze in the atmosphere at that site, providing brighter and sharper images.
B) The winds are lighter at the better site, thereby producing less telescope vibration.
C) There is less air turbulence at that site, causing less twinkling and blurring of the images.
D) There are more clear nights at that site.