You are presented with the question "when is timing critical?" and the following answer choices: 
- for gravity
 - Warm air moving into cold air
 - earth's orbit
 - when sperm and egg fuse
 - flow of electrons
 - earth's rotation
 - Earth revolving around the sun
 - movement of tectonic plates

Now knowing that fertilization occurs when a sperm and an egg fuse and fertilization As far as fertilization, timing is critical, choose the best answer.


 

    when sperm and egg fuse     