1. When did stromatolites appear on Earth?
Archean
Hadean
Mesozoic
Proterozoic

How does pregnancy begin? a) a placenta forms in the uterus b) a sperm reaches an egg in the fallopian tubec) the blastocyst differentiates into two cell types d) contractions begin in the uterine wallapex

How does food concentration affect yeast activity

Which gas prevents the spread of fire and extinguishes it? is a type of compressed gas that prevents the spread of fire and extinguishes it.