Answer the following question given this paragraph: 

After the ball of cells reaches the uterus, it fixes itself to the side of the uterus. This is called implantation . It usually happens about a week after fertilization. Now the implanted ball of cells is ready to continue its development into a baby boy or girl.


Q: What is the process of oxidized iron that has been exposed to air and water?

 Choices:

- bark

- crust

- crystallization

- rust

A:
rust
