(1). for gravity
(2). Warm air moving into cold air
(3). earth's orbit
(4). when sperm and egg fuse
(5). flow of electrons
(6). earth's rotation
(7). Earth revolving around the sun
(8). movement of tectonic plates

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, when is timing critical?
Answer:
(4).