It involve both experimentation and repeated observations
The answer to this question would be A:growth factors
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The gravitational force is being felt as the Earth pulls the train downwards using the two forces. The coils that provide the forces releases an electrical current which makes an electromagnetic field. It repels the materials with an equal force that goes up as well.
Matter can be recycled, but energy cannot. Also, only 10% of the energy is passed through each trophic level.
