This image shows the rio grande, a river that separates texas and mexico.
which type of boundary is this?
a) cultural
b) geometric
c) physical
d) religious

The Rio Grande is an example of a physical boundary, other examples exist where boundaries correspond with any rivers, mountains, deserts, or really any defining body of water. A cultural boundary would be the borders between countries or regions that also corresponds to differences in ethnicity, language, and cultural norms, a geometric boundary would be the border between countries without regard to physical boundaries, such as the US-Canada border, and religious boundaries would be state boundaries that also correspond to religious differences, such as between primarily Hindu India and primarily Muslim Pakistan. 

Why the planet stays in orbit is because of inertia (object stays in motion until a force changes it’s motion) also the atmosphere and gravity!