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Kevin uses an elevator to lift 10 boxes of 10 kg each up to a fourth floor at
a height of 18 m. When he's finished, Kevin notices that the elevator
motor and brakes as well as the air around them are hot. A device
connected to the elevator indicates that 24,100 J of energy were
consumed. Use g = 9.8 m/s².
Which of the following statements is the best possible explanation for the
movement of energy?