Answer the following question given this paragraph: 

The first horizon is the “A“ horizon. It is more commonly called the topsoil . The topsoil is usually the darkest layer of the soil. It is the layer with the most organic material. Humus forms from all the plant and animal debris that falls to or grows on the ground. The topsoil is also the region with the most biological activity. Many organisms live within this layer. Plant roots stretch down into this layer. The roots help to hold the topsoil in place. Topsoil is needed to grow most crops ( Figure below ).


Q: What is the "a" horizon commonly known as?

 Choices:

- sediment

- core

- crust

- topsoil

A:
topsoil
