Which layer is the oldest and how do you know

c

Explanation:

because it has made its way to the bottom meaning its been their longer

its not a b/c it on top and hasn't had enough time to break down

its not b b/c well theres nothing their

layer C

Explanation:

layer C is the oldest because sand piled up on top of each other century after century or year after year.

hope this helps

The answer is true  good luck and have a nice day