A、It is being raised each year. B、It remains constant. C、It will reach sea level in 9,000 years. D、It is being lowered gradually

The ordinary raindrop is a mighty earthmover with sufficient strength to cut rock. When rainwater collects on the surface of the ground, some of it evaporates and some of it sinks into the earth. The remainder begins to flow downhill, commencing its lengthy journey from brook to stream to lake, or to a river that will carry it to the sea.
    As water flows along the ground, it picks up sand, pebbles, even boulders. It uses them to gnaw at the sides and bottoms of its channel, gradually loosening more earth.
    By this process enormous amounts of mud and rock are moved from the land to the sea. Each year the Mississippi River carries 730 million tons of solid matter into the Gulf of Mexico.
    This constant hauling of land into the sea is lowering the United States’ average height above sea level at a rate of about one foot every 9,000 years. If erosion continues at the same rate, the United States will be worn completely down to sea level in about 23 million years.
What does erosion by water result in?