Fossils layers 175 mya, 250 mya, 400 mya, 450 mya

450 mya, is where you would find fossils.

Explanation:

The first layer would be newer organismis that haven't quite decayed yet.The second and third layer is where the bones of the dead organasims lie, they havent quite been through the whole process yet, but they are nearly complete just about 100,000 years left.The fourth layers have been through alot, and have been there for many many years we can say abot 100,000,000 years.They are ready to become a fossil fuel.

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds