Explanation:
in my opinion I'd say it's gravity because it's making the book stay in it's one and only position pulling the book at the center of the desk
A book is sitting on a desk is due to the Gravitational force of Earth Gravity so D. the downward force the Earth pulls on the book.
The force of attraction between all masses that is objects in the universe, especially the attraction of the Earth's mass for bodies near its surface.
Einstein said that the shape of spacetime is responsible for the rise of force that we experience as gravity. A concentration of mass, such as the Earth or sun, bends space around it like a rock bends the flow of a river.
Learn more about Gravity here
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