An egg requires warmth to survive, and inside of the eggs, the embryos grow by shedding their inelastic shell or exoskeleton. Given these facts, what inside of a shell requires warmth to survive among the following options:
- oysters
 - abalone
 - bullfrogs
 - sand
 - beads
 - water
 - embryos
 - Echinoids


 

       embryos  