* A. Friction: Friction is a force that opposes motion between two surfaces in contact.

* D. Gravity: Gravity is the force of attraction between any two objects with mass.

Let's break down why the others aren't natural forces:

* B. Chemical: Chemical reactions involve the rearrangement of atoms and molecules, driven by the breaking and forming of chemical bonds. While these reactions are a part of nature, "chemical" itself isn't a force in the way friction or gravity are.

* C. Electromagnetic: This is a fundamental force of nature, but it's not listed as a specific "force" like friction or gravity. Electromagnetism encompasses forces like electric and magnetic forces.

In summary: Friction and gravity are natural forces that directly influence the motion and interactions of objects in the universe.