Green spaces are important in cities because they reduce the ambient amount of
a) oxygen
b) hydrogen
c) nitrogen
d) carbon dioxide

carbon dioxide

Explanation:

Green spaces are important in cities because they reduce the carbon dioxide concentrations that are produced most by vehicular activity. At the same time, these green spaces provide the city with an invaluable source of oxygen that otherwise would not be found in city limits due to pavimentation and lack of naturally forested areas or green spaces.

put the answers   and i could answer.

i think that the answer is c