Read the following passage and answer the question: 'The Cambrian explosion, a period of rapid evolutionary change, saw the emergence of most major animal phyla. However, the fossil record from this era is incomplete, leaving scientists to debate the exact timeline and causes. Some theories point to environmental factors, such as an increase in oxygen, while others suggest genetic innovations were the primary driver.' Which of the following is the main idea of the passage?
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A
The Cambrian explosion was solely caused by an increase in atmospheric oxygen.
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B
The fossil record from the Cambrian period is perfect and without any gaps.
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C
Scientists fully agree on the causes and timeline of the Cambrian explosion.
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D
The Cambrian explosion was a significant evolutionary event with causes that are still debated by scientists.