Here's why:
* Crust: The Earth's outermost layer, ranging from about 5km (oceanic crust) to 70km (continental crust) thick.
* Mantle: Lies beneath the crust and extends to a depth of about 2900km.
* Outer Core: Located between 2900km and 5150km deep.
* Inner Core: The Earth's innermost layer, extending from 5150km to the center of the Earth (about 6371km).
Since 20km falls within the range of the crust's thickness, that is the layer you're looking for.
