- To transport can mean to carry
- Transporting is a kind of function
- Therefore, carrying is a kind of function

- Carrying is a kind of function
- Red blood cells carry oxygen
- Hence, the function of red blood cells is to carry oxygen

Therefore, the function of red blood cells is to carry oxygen
What is the primary job of red blood cells?
- Red blood cells carry oxygen 
- To transport can mean to carry 
- Transporting is a kind of function