Which layer is the oldest and why? in one sentence

The diagram shows Layer A cutting through Layer B and Layer C, with Layer B being below Layer C.

Layer C is the oldest because it is the bottom layer in the diagram, indicating that it was formed first.

Based on the information given, Layer C is the oldest because it is the layer that has been cut through by both Layer A and Layer B.

Layer C is the oldest because it is the layer that has been cut through by both Layer A and Layer B, indicating that it was there first and then subsequently covered by the other layers.