Which function is decreasing?

a: outdoor temperature as a function of time.
b: the dow jones industrial average as a function of time.
c: air pressure in the earths atmosphere as a function of altitude.
d: world population since 1900 as a function of time.

The function which is decreasing is C. Air pressure in the earths atmosphere as a function of altitude.
The higher you go, the less air pressure there is because of the fewer air molecules at such altitudes. 
This means that the remaining functions are either constant or increasing.