Pick from:
[a]. for gravity.
[b]. Warm air moving into cold air.
[c]. earth's orbit.
[d]. when sperm and egg fuse.
[e]. flow of electrons.
[f]. earth's rotation.
[g]. Earth revolving around the sun.
[h]. movement of tectonic plates.

You are presented with the quiz "when is timing critical?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "fertilization occurs when a sperm and an egg fuse" and "fertilization As far as fertilization, timing is critical". So, what's the best answer to the question?
Answer:
[d].