Answer:
The correct answer is D)
Explanation:
Oxygen is a chemical element which plays a major role in sustaining complex life, and that is mostly produced on Earth through the photosynthesis of plants. Without the constant replenishment of oxygen provided by this biological process, oxygen would be extremely rare in the Earth's atmosphere, as it is too volatile.
This is exactly the situation which the Earth was going through during the first billion years of its existence. Early Earth was mostly devoid of life, so oxygen concentration on the atmosphere was practically zero. The appearance of the first few unicellular beings a few hundred million years later which produced oxygen through photosynthesis had little effect on the concentration of this gas, as most of it oxidized in the atmosphere as soon as it was produced. However, as life diversified and expanded, more and more photosynthetic organisms developed and flourished. A little less than a billion years ago, oxygen concentrations were rising enough to allow the development of more complex life. Today, oxygen concentration in the atmosphere sits at around 21%.
