In order to survive, humans require a minimum of one-third of the oxygen content at sea level.Using the figure below, what was the most recent point in Earth's history at which a human could (barely) survive unaided on the surface? 
A) 250 million years ago
B) 600 million years ago
C) 1 billion years ago
D) 3 billion years ago
E) 10 billion years ago.
Correct Answer:
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