Force is a term that
a unchanging over time - like balancing
b movement around a center point - like throwing
c strength used in an action - like lifting weights
d speed over time - like running

Option d , speed over time - like running

Explanation:

Force is mathematical terms in defined as a product of mass of an object and it acceleration.  

Acceleration is caused when speed changes over time.  

Among all the given options, in option D the speed over time will cause acceleration and thus produce a force.  

Force being a vector quantity required change of speed in  some direction. While running , both quantity and direction can be assumed to be changing.  

Thus, option D is correct.

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before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.