In a very young planet, called a protoplanet, elements are distributed evenly
throughout. As the protoplanet ages, the elements separate, leading to the formation
of layers with different element concentrations. Some of these elements are listed,
along with their atomic masses and laboratory densities.

Select the elements that would be found in high concentrations in the center of the planet.

Aluminum
Calcium
Iron
Nickel
Silicon
Sodium