题目内容

Water ____________ oxygen and hydrogen..

A. contains B. is contained C. includes D. included

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根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。选项中有两处为多余选项。

Active ＆ Passive Transport

　　Recall that all living things are made up of tiny units called cells, and that -cells are surrounded by a covering called a membrane (膜)．The membrane allows things to move in and out of the cell, and it also controls what can get through．　　1　　 They must have water, oxygen, sugar, sodium, potassium (钾) , and a variety of other minerals．The body has to have a delivery system to get these materials where they need to go．In addition, every cell produces waste materials that it needs to get rid of．　　2　　

　　Just like cars, trucks, and other vehicles form a transport system for the pick up and delivery of materials out in the world, the body has a transport system, too．　　3　　

　　Passive transport is easiest for the cells because they don't need to use any energy to make it happen．Diffusion (扩散) is the simplest and most common form of passive transport．

　　During the diffusion process, tiny particles (微粒) of the materials that need to be delivered to the cells are spread through either a gas (like oxygen) or a liquid (like water)．　　4　　Osmosis (渗透作用) is a unique form of passive transport that allows water to pass through the cell membrane, but keeps most other materials out．

　　Active transport is used when the concentration of materials inside the cell is high, and the cell needs to push materials out to areas of low concentration of the materials．Unfortunately, this kind of transport requires the cell to work against the natural direction of diffusion, so the cell uses up energy．　　5　　 They pump sodium out of their cells and pull potassium in．Although they must use energy in this process, doing so allows them to thrive (成长)．

A．Areas with high concentrations of the materials slowly diffuse them to areas of low concentration．

B．Cells need many kinds of materials in order to survive．

C．One example of this is the way that nerve cells work．

D．There are two kinds of transport：active transport, and passive transport．

E．Passive transport is much more essential than active transport．

F．The body has to have a method of waste collection and disposal．

G．Then the cell usually needs extra energy to make up for it．

Life on Mars could become a reality and it could happen in your lifetime.

A welcoming planet

Scientists say Earth’s neighbor Mars, a bright red planet about half Earth’s size, is the most likely to support human life. Mars even has frozen water on its surface.

Since the late 1990s, NASA has been exploring Mars using remote-controlled vehicles（装置）. Most recently Curiosity, a car-size vehicle, traveled through space on an unpiloted spacecraft(航天器) and landed on Mars in August 2012. Directed by NASA scientists , the vehicles move on the surface, taking pictures , collecting and analyzing soil, and looking for signs of life.

But what about human explorers? Plans are already in the works to send astronauts to Mars as soon as the mid-2030s.

_____________________________________

But before you start packing your bags, let’s consider the challenges. For starters, Mars is far away. Just getting there could take up to 10 months.

Scientists already know that time away from Earth’s gravity harms the human body. Bones and muscles get weaker. The body produces less blood. What damage would months and months of living in space do?

And then there is the matter of water, oxygen, food and fuel. Scientists will have to find solutions to these problems, or the first humans on Mars won’t survive very long in their new home.

Tiny Dangers

There’s another tinier risk. It’s so tiny that you can’t even see it: germs.

Some scientists believe that our germs could pollute the whole planet of Mars. Potentially killing Martian life before we have the chance to discover it. Worse, there is a small but terrifying chance that any microscopic life already there might be harmful to us .

Worse still, if any of those Martian germs(火星细菌) were brought back to Earth, the result could be disastrous. Animals, plants, and people could be wiped out.

Worth the $$$?

A more practical concern is the cost. The price could approach $ 1 trillion(万亿). How can we justify spending that much when so many problems—poverty, disease—could use the cash here on Earth?

1.Which of the following is TRUE according to the “A welcoming planet” part?

A. Mars is a little bigger than Earth.

B. There are flowing rivers on Mars.

C. People haven’t been to Mars so far.

D. Scientists have discovered signs of life on Mars.

2.Which of the following can best fill in the blank in the passage?

A. Living in space. B. Limited resources.

C. Extreme conditions. D. Interesting challenges.

3.By “Tiny dangers” the author means ________________.

A. there is no serious danger

B. people won’t be in any danger

C. it’s difficult for people to realize the danger