Write the next sentence in this paragraph:

How to build a concrete foundation
Choose the type of foundation to build.
The type of foundation depends on the area of the foundation will be located on and the type of structure that will be placed on it.
Shallow foundations are built on level ground and hard surfaces.

OPT:
(A). They need to be elevated with a railing or a wire gate before entering the area. Medium foundations use stands or benches built on rocks at the bottom of the drop off area.
(B). Low and compact foundations are built on concrete ground and have minimal concrete features. Choose the level of the site where you want your foundation to go.
(C). Shallow foundations are no more than 3 feet (91.44 cm) deep and are used primarily for smaller, simpler projects like holding patio furniture, a fountain, or an air conditioning unit. Deep foundations are used for more complex projects.
(D). Concrete will require some compaction to accommodate built foundations. Concrete will require some compaction, otherwise the area won't be properly tilled.
(C).