Problem: What is the missing first step of the following process: - Is exposed to air - A chemical reaction occurs - The iron starts to oxidize - The iron starts to rust.

A: Iron is exposed to oxygen

Problem: What is the missing first step of the following process: - Embryo is C - Embryo grows and develops into a fetus inside the woman's womb - The fetus is born - Begins to grow and develop through infancy and childhood - In the teenage years the human experiences puberty - Reaches sexual maturity - At 18 the human becomes an adult - Adult may procreate continuing the cycle.

A: A man and woman mate

Problem: What is the missing first step of the following process: - The fault lines grow over time - The rocks on each side of the fault lines rub against the rocks on the other side of the fault line - The rocks put pressure on each other - The rocks break from the pressure - The breaking of the rocks releases a lot of energy - The energy causes seismic waves - The seismic waves make the earth shake.

A: Fault lines appear underground

Problem: What is the missing first step of the following process: - The passenger arrives early at the airport - The luggage is checked - The passenger waits in line - The passenger shows their identification and gets a boarding pass - The passenger goes to their gate - The passenger gets in line when it's time to board - The person boards the plane - The plane arrives at the destination - The passenger gets off the plane at their destination.

A:
An airline reservation is made