Why are atmospheric co2 concentrations low in the northern hemisphere in summer and high in the winter? see section 53.3 (page 1130) ?

Because in the Northern Hemisphere there are usually four seasons. During summer time, trees are most "active" in terms of photosynthesis given the intense sunlight. Thus more CO2 is absorbed from the atmosphere. During winter time, trees usually shed their leaves for the winter and thus photosynthesis is very low and less CO2 is absorbed from the atmosphere.

A.(nh4)(2co3)is the correct answer 

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