The ordinary raindrop is a mighty earth mover with sufficient strength to cut rock. Wh

en 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 form. 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 bottom 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 9000 years. If erosion continues at the same rate, the United States will be worn completely down to sea level in about 23 million years.

1.Erosion by water results in _______.

A、wearing down the soil

B、building up land

C、enriching the soil

D、both B and C

2.Flowing water gradually makes its channel ______.

A、narrower

B、deeper

C、wider

D、both B and C

3.Implied but not stated: ________

A、Millions of years ago the United States was higher than it is now

B、Great amounts of land are moved to the sea by rivers

C、The water that sinks into the earth also reaches the ocean

D、The Gulf of Mexico will be filled up in about 23 million years

4.The best title for this selection is ________.

A、How Water Destroys Mountains

B、How Rivers Help Us

C、The Average Height of the United States

D、The Mighty Raindrop

5.The average height of the United States above sea level ___.

A、is being raised each year

B、remains constant

C、will reach sea level in 9000 years

D、is being lowered little by little