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Cambridge 14, test 2, task 1. The diagram illustrates how the electricity is produced in a hydroelectric power station. The hydroelectric power station is made up a dam behind which there is high-level reservoir. It also consists of a generator, reversible turbines, and low-level reservoir. Once river flows to dam, it collects behind the dam to make a high- level reservoir. Throughout day water goes to power station through intake which is closed during night. In power station , reversible turbines rotates by water left from intake. Then this water enters low- level reservoir. One of the functions of turbines is to operate generator over day times. The electricity produced by generator is transmitted to national grid by power lines. At night, the water gathered in low- level reservoir is pumped into high-level one by reversible turbines. Overall, this diagram explains how the power of water can be harnessed to generate electricity in efficient way that water is not wasted.