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Tidal Power the Gravitational Pull of the Moon and the Sun, as Sun

question 142

Multiple Choice

Tidal Power
The gravitational pull of the moon and the sun, as well as Earth's rotation, create the tides-the rise and fall in sea levels. In some places, tidal action can cause water levels-especially those near the shore-to vary up to forty feet. Today, the awesome power of the tides is used to generate electricity. The two main types of tidal power systems are tidal barrages and tidal turbines.
Tidal barrages use a structure similar to a dam. The structure, called a barrage, is located across an inlet of an ocean bay or lagoon. Massive gates attached to the barrage open and close to control water levels and flow rates. They allow the tidal basin to fill with water on the incoming flood current. This water is then emptied through an electric turbine system on the outgoing current. As water races through the turbine system, electricity is generated.
Tidal turbines look and operate quite similar to wind turbines. But because water is about 800 times denser than air, tidal turbines must be much sturdier and heavier. These power generation units are permanently attached to the sea floor in areas with a strong tidal flow. They operate as the ocean's current spins past massive blades. The blades are attached to turbines that produce electricity. Systems in Scotland and South Korea have tidal turbines that are capable of producing up to 1.5 megawatts of electricity-enough electricity to power roughly 1,500 homes.
But tidal power is not without some drawbacks. Scientists think that the electromagnetic waves generated by the tidal turbines could disrupt ocean life. In particular, scientists are worried that they might affect the migration patterns of aquatic animals. But already, a workaround has been suggested-the turbines are simply turned off when certain migrating sea mammals approach them. Another drawback of tidal power is the high cost of construction. They are, for example, much more expensive to install than wind or solar power facilities. Still, tidal power facilities are durable, lasting as much as four times as long as wind or solar power plants.
Tidal power emits zero greenhouse gases, so it is an extremely clean power source. It takes up relatively little space, too, when compared to other green energy projects. For example, the Sihwa Lake Tidal Power Station in South Korea was easily added to an existing 12.5 kilometer-long seawall. Windfarms and even solar farms, on the other hand, can take up vast tracts of land, sometimes amounting to hundreds of square kilometers. Another benefit of tidal power is that the power it generates is extremely predictable. Obviously, the gravitational pull of the sun and moon are not going to cease any time soon. The benefits of tidal power seem to outweigh its drawbacks. Clearly, it's time to produce more of these electricity-generating facilities wherever possible.
What is the meaning of "emits," as used in the fifth paragraph of the passage?

Definitions:

Parenchyma Cells

A type of plant cell known for its role in photosynthesis, storage, and tissue repair and regeneration.

Horizontal Pathway

A gene transfer method in which genetic information moves between organisms in a manner other than traditional reproduction.

Soil

The top layer of earth in which plants grow, composed of organic matter, minerals, gases, liquids, and organisms that together support life.

Pore Spaces

Voids or gaps found within soil or rock, where air or fluid can be stored.