A) If both the assertion and the reason are true and the reason is a correct explanation of the assertion
B) If both the assertion and reason are true but the reason is not a correct explanation of the assertion
C) If the assertion is true but the reason is false
D) If both the assertion and reason are false
E) If the assertion is false but reason is true
Correct Answer: B
Solution :
The photoautotrophs utilise the solar energy absorbed by them in systhesizing organic compounds. The process is called photosynthesis. The first photoautotrophs were bacteria-like. They were anaerobic and utilised hydrogen from sources other than water. Therefore, no oxygen was evolved and the atmosphere remained reducing in nature. It is suggested that simple one celled organisms some what similar today's cyanobacteria were present on earth about 3600 million years ago. These organisms utilised water as hydrogen donor and evolved oxygen. As a result the atmosphere became richer and richer in oxygen.
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