Earth's first atmosphere was rich in water vapor but poor in free ...

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Earth's first atmosphere was rich in water vapor but poor in free oxygen. The atmosphere became more oxygenated with the
 
  A) development of the ozone layer.
  B) emergence of stromatolites that developed a simple version of photosynthesis.
  C) emergence of gases from volcanic eruptions.
  D) both A and B.

Question 2

What was the one most important event in the Precambrian?
 
  What will be an ideal response?

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Answer to Question 1

Answer: B

Answer to Question 2

Answer: The accumulation of free oxygen in the atmosphere. This allowed the ozone layer to form and allowed life to emerge.