- Oceans cover 70% of the surface of the earth
- Most means over 50% of something
- So, oceans cover most of earth's surface

- Oceans cover most of earth's surface
- An ocean is a kind of body of water
- Hence, water covers most of earth's surface

Therefore, water covers most of earth's surface
Q: Most of Earth's surface is covered by
Context:
- An ocean is a kind of body of water 
- Oceans cover 70% of the surface of the earth 
- Most means over 50% of something