Which statement best describes a pumped-storage hydropower system? 

A)  It may vary in output during different seasons. 
B)  It is used to produce power during peak periods. 
C)  It involves the use of large dams. 
D)  It depends on stream flow to control power generation.

A pumped-storage hydropower system is used to produce power during peak periods when there is high demand for electricity. The system involves pumping water from a lower reservoir to an upper reservoir during times of low energy demand and releasing it through turbines during peak demand periods to generate electricity. This helps to balance the grid and ensure a reliable supply of electricity during times of high demand. The system's output can remain consistent regardless of the season, making option A incorrect. Large dams are not necessarily required for this system, making option C incorrect. The system relies on the stored water in the upper reservoir, not stream flow, to generate power, making option D incorrect.