Text Structure and Purpose

SAT Reading and Writing· difficulty 3/5

Tides on Earth are caused mainly by the gravitational pull of the Moon, which attracts the ocean's water more strongly on the side of Earth facing it. The water on the opposite side experiences a slightly weaker pull and therefore "lags behind," producing a second high tide each day. Because Earth rotates while the Moon orbits, most coastlines experience two high tides and two low tides every roughly twenty-five hours.

What is the main purpose of the text?

  • A

    To argue for a new theory of tidal generation.

  • B

    To describe how tides affect marine ecosystems.

  • C

    To explain the gravitational mechanism behind Earth's daily tides.

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  • D

    To compare lunar and solar tides.

Explanation

The text walks through the gravitational mechanism producing daily tides. B is correct.

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