What is force?
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In physics, a force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity, i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity.

→A force is a push or pull upon an object resulting from the object's interaction with another object.

→Whenever there is an interaction between two objects, there is a force upon each of the objects.

the correct answer is:

a population changes in response to a change in the environment.