How often does oxygen cycle through the atmosphere?
a. about every 1000 years
b. about every 1500 years
c. about every 2000 years
d. about every 2500 years

oxygen is a constant 20 percent throughout the atmosphere. Even in space but the air is so thin in space that the oxygen would have to be compressed to be able to breathe it. Here on earth at sea level the atmospheric pressure is 14.7 pounds per square inch (PSI) making it possible for your body to inhale enough usable oxygen for you to breathe. As you go higher the pressure is less so even though the oxygen is still 20% of the atmosphere there is less atmosphere for you to breathe. THE ANSWER IS A