Fact 1: Breathing is when a gill converts from oxygen in water into oxygen in blood.

Fact 2: Many people confuse breathing with respiration, but breathing is just the first step of respiration.

Given the two facts above, answer the question "what is it when a gill converts from oxygen in water into oxygen in blood?" with the following options: 
- fertilization occurs
 - prolactin release
 - preventing heat transfer
 - first step of respiration
 - last step of digestion
 - flow of electrons
 - death
 - fish


 

    first step of respiration     