阅读理解。 Lichens (地衣) are difficult to see

题目：

阅读理解。

     Lichens (地衣) are difficult to see. They don't move. They often mix into the living place. You might not

even recognize one if you are looking right at it.

     Lichens live on rocks, branches, houses, even metal street signs. You can find these colourful organisms

(有机体) almost everywhere-from deserts to rainforests, from Antarctica to Africa. They've traveled in outer

space, and some scientists think there might even be lichens on Mars.

     What you probably don't realize is that a lichen is more than a single thing. It consists of two different types

of living organisms: a fungus (菌类) and an alga (藻类). Neither of these organisms is a plant, so the lichen isn't

a plant either.

     Through photosynthesis (光合作用), the alga absorbs the sun's energy to make food for the fungus, which

is a place for the alga to live. However, the alga cannot leave the fungus.

     Around the world, scientists have found tens of thousands of types of lichens. Many may not even have

been discovered yet. As scientists continue to find new kinds of lichens, they are also working to understand

how the lichens are connected to one another. By putting together a lichen family tree, they hope to understand

why so many different types of lichens have grown in so many places around the world.

     Many researchers are trying to understand the basic facts of the organisms and their connections.

Researchers are also using lichens to examine the health of the environment.

1. What do we know about lichens according to the passage?

A. They will die if they leave the earth.

B. There are more lichens than plants on the earth.

C. They are similar to the environment around them in colour.

D. They are too small to be easily seen by people.

2. What is the function of the alga in lichens?

A. It produces food for the fungus.

B. It offers a living place for the fungus.

C. It makes the lichen a plant.

D. It can protect the fungus.

3. We can know from the passage that _____.

A. lichens can be used to better examine the environment

B. lichens don't have any differences in colour or size

C. lichens don't use photosynthesis like a green plant

D. scientists have discovered every kind of lichen