The decomposition of marine plants that depletes dissolved oxygen
If later is was 100 million years ago, later b would be 200 million years ago (because it is 100millions years prior to a). And therefore layer c must b 300 million years ago.
<span>These organisms obtain energy and carbon from the physical environment and use it to produce their food. They make sugars and other compounds using sunlight as a source of energy and carbon dioxide as a carbon source. The evolutionary advantages for these organisms are the periodic renewal of atmospheric O2 for all living beings. The production of organic matter and food webs of any ecosystem. Without them, there is an extinction of all living organisms due to the non-renewal of atmospheric O2 and the excessive accumulation of CO2 in the atmosphere.</span>
I think the answer is F<span>ossil shells on top of mountains</span>
