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The key to this question is to carefully think about the first sentence and to apply your understanding of KE and PE to the situation. See A Very Fine Detail section.
Kinetic energy depends upon mass and speed. See Dictionary section. A constant speed means a constant kinetic energy. And any change in speed will lead to a corresponding change in kinetic energy.
Potential energy depends upon mass and height. See Dictionary section. A constant height means a constant potential energy. And any change in height will lead to a corresponding change in potential energy.
The total mechanical energy is the sum of the kinetic and potential energies. See Dictionary section. If either KE or PE increases while the other remains constant, then the sum (total amount of mechanical energy) will obviously increase. If either KE or PE decreases while the other remains constant, then the sum (total amount of mechanical energy) will obviously decrease.
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