<span> the atmosphere holds about 21 per cent oxygen. Over the Earth’s 4.6 billion year history, oxygen did not appear in the atmosphere until perhaps about 2.5 billion years ago. Since then, oxygen levels have fluctuated in tandem with global geological and biological events, such as mass extinctions.</span>
Answer:
1
Explanation:
The cell membrane is the protective, semi-permeable membrane surrounding a cell and its contents.
Explanation:
the physical and chemical properties of an element are periodic functions of their atomic number.
