[单项选择]（　） most of the earth's surface is covered by water, fresh water is very rare and precious.
A. As
B. Once
C. If
D. Although

[单项选择]Just over two-thirds of Earth’s surface is covered by wafer, _________ more than 98 percent of this water is contained in the oceans.
A. with
B. which
C. and
D. resulting

[单项选择]It can be seen from the passage that the earth's supply of natural medicines
A. may never be exhausted.
B. may be dropping rapidly.
C. is surprisingly big.
D. is as rich-as ever.

[单项选择] How Global Warming Works Gases in the Earth’’s atmosphere act like glass in a greenhouse-trapping heat and making life on Earth possible. But there is a delicate balance. Burning coal, oil and natural gas increases atmospheric concentrations of these gases. Over the past century, increases in industry, transportation, and electricity production have increased gas concentrations in the atmosphere faster than natural processes can remove them leading to human-caused warming of the globe. The Sources Of Global Warming The major source of global warming is carbon dioxide pollution from power plants, auto mobiles, and industry. Another source is global deforestation. ●Power plants are responsible for more than a third of U. S. CO2 emissions, yet there are no caps on CO2 emissions from power plants or any other industry. ●Gas guzzling cars and light trucks are also responsible for a third of U.S. CO2 emissions. Current regulations allow fo
A. Y
B. N
C. NG

[单项选择]
Most of the time, the ground feels solid beneath our feet. That’s comforting. But it’s also misleading because there’s actually a lot going on underground. Masses of land (called plates)slip, slide, and bump against each other, slowly changing the shape of continents and oceans over millions and billions of years.
Scientists know that Earth formed about 4.5 billion years ago. They also know that our planet was hot at first. As it cooled, its outermost layer, called the crust (地壳), eventually formed moving plates. Exactly when this shift happened, however, is an open question.
Now, an international group of researchers has an answer. They’ve found new evidence suggesting that Earth’s crust started shifting at least 3.8 billion years ago. The new estimate is 1.3 billion years earlier than previous ones.
Not long before 3. 8 billion years ago, lots of asteroids (小行星) were hitting Earth, keeping its crust in a ho
A. Right
B. Wrong
C. Not mentioned