) The most important source of the free oxygen in our atmosphere is: A) green plants that carry on photosynthesis. B) deforestation. C) volcanic degassing. D) the dissociation of water vapor in the upper atmosphere.

Option A) green plants that carry on photosynthesis

Explanation:

Photosynthesis is the process by which green plants generate their food from simple inorganic molecules like atmospheric carbon dioxide and water in the presence of sunlight trapped by chlorophyll, giving off free molecules of oxygen gas as by-product into the atmosphere.

The equation is shown below:

6CO2 + 6H2O --> C6H12O6 + 6O2

Thus, everytime plants undergo photosynthesis, they release 6 moles of free oxygen gas into the atmosphere.

relative dating involves things like higher layers are most recent than rocks lower in the sequence. this can be extended to the known sequence of fossils. rocks containing rabbit fossils are later than those containing dinosaur fossils.  

absolute dating methods mainly use radioactive elements that have known rates of decay and produce recognizable products. for example potassium 40 decays to produce argon. once a rock has solidified the argon is trapped. by measuring the argon and potassium0 you can get an accurate number for the age.  

some other methods are good for archaeological ages. dendrochronology involves recognizing the sequence of tree rings. the orientation of magnetized particles in baked clay can be used to date a fire.

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