Fact 1: Fertilization occurs when a sperm and an egg fuse.

Fact 2: Fertilization As far as fertilization, timing is critical.

Given the two facts above, answer the question "when is timing critical?" with the following options: 
- 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


 

    when sperm and egg fuse     