If the first step of respiration is when a gill converts from oxygen in water into oxygen in blood, then what is it when a gill converts from oxygen in water into oxygen in blood?

Answer choices:
- fertilization occurs
 - prolactin release
 - preventing heat transfer
 - first step of respiration
 - last step of digestion
 - flow of electrons
 - death
 - fish

 

    first step of respiration      