Order the following layers of the Earth from least dense to most dense.

a) Crust, Upper Mantle, Lower Mantle, Outer Core, Inner Core
b) Upper Mantle, Lower Mantle, Crust, Inner Core, Outer Core
c) Crust, Outer Core, Upper Mantle, Inner Core, Lower Mantle
d) Crust, Upper Mantle, Lower Mantle, Inner Core, Outer Core