- "Something hard can be used for protection " is not a useful statement here

- Oceans contain most of earth 's water
- Water is made of hydrogen and oxygen
- Therefore, ocean water is made of hydrogen and oxygen

- Ocean water is made of hydrogen and oxygen
- Oxygen is more common than hydrogen on earth
- So, oxygen is the most common element found in the compounds that make up ocean water

Therefore, oxygen is the most common element found in the compounds that make up ocean water
[Question]What is the most common element found in the compounds that make up ocean water?
[Context]- Oxygen is more common than hydrogen on earth 
- Oceans contain most of earth 's water 
- Something hard can be used for protection 
- Water is made of hydrogen and oxygen