9. The air pressure in winter is higher than that in summer (). A, low b, high c, quite

A, the standard atmospheric pressure is the same everywhere, so A is wrong;

B, atmospheric pressure decreases with the increase of height, atmospheric pressure is related to height, so b is wrong;

CD and atmospheric pressure are not fixed, but they are related to altitude and weather. Generally, the air pressure is higher in sunny days than in cloudy days, and higher in winter than in summer. So, C is wrong and D is right.

Therefore, choose: d.