多选题Consider each of the choices separately and select all that apply.　 Which of these can be inferred from the fact that the French built orangeries for certain tropical plants?AGreenhouses may be used to keep plants at a moderate temperatureBThe climate

A. Only router can split boundaries(horizons)between concentric networks.

B. All distance vector protocols require fall routes that may cause momentary loops as the topology changes.

C. Networks can only remain fully converged if all information about routes is sent out all active interfaces.

D. Information about a route should not be sent back in the direction from which the original update came.

E. Each AS must keep routing tables converged to prevent dead routes from being advertised across the AS boundary.

29． Why are green plants and grass helpful?

A． Because they can hold the water．

B． Because they can stop the wind from blowing the earth away．

C． Because they can keep the earth wet．

D．All the above．

C

Plants can’t communicate by moving or making sounds, as most animals do. Instead, plants produce volatile compounds, chemicals that easily change from a liquid to a gas. A flower’s sweet smell, for example, comes from volatile compounds that the plant produces to attract insects such as bugs and bees.

Plants can also detect volatile compounds produced by other plants. A tree under attack by hungry insects, for instance, may give off volatile compounds that let other trees know about the attack, In response, the other trees may send off chemicals to keep the bugs away- or even chemicals that attract the bugs’ natural enemies.

Now scientists have created a quick way to understand what plants are saying: a chemical sensor(传感器) called an electronic nose. The “e-nose” can tell compounds that crop plants make when they’re attacked. Scientists say the e-nose could help quickly detest whether plants are being eaten by insects. But today the only way to detect such insects is to visually inspect individual plants. This is a challenging task for managers of greenhouses, enclosed gardens that can house thousands of plants.

The research team worked with an e-nose that recognizes volatile compounds. Inside the device, 13 sensors chemically react with volatile compounds. Based on these interactions, the e-nose gives off electronic signals that the scientists analyze using computer software.

To test the nose, the team presented it with healthy leaves from cucumber, pepper and tomato plants, all common greenhouse crops. Then the scientists collected samples of air around damaged leaves from each type of crop. These plants had been damaged by insects, or by scientists who made holes in the leaves with a hole punch (打孔器).

The e-nose, it turns out, could identify healthy cucumber, pepper and tomato plants based on the volatile compounds they produce. It could also identify tomato leaves that had been damaged. But even more impressive, the device could tell which type of damage – by insects or with a hole punch – had been done to the tomato leaves.

With some fine tuning, a device like the e-nose could one day be used in greenhouses to quickly spot harmful bugs, the researchers say. A device like this could also be used to identify fruits that are perfectly ripe and ready to pick and eat, says Natalia Dudareve, a biochemist at Purdue University in West Lafayette, Ind. who studies smells of flowers and plants. Hopefully, scientists believe, the device could bring large benefits to greenhouse managers in the near future.

49. We learn from the text that plants communicate with each other by______.

A. making some sounds B. waving their leaves

C. producing some chemicals D. sending out electronic signals

A

Why do plants grow in some places and not in others? Why does some land have so much growing on it，while other land has almost no plants growing on it at all?

To grow plants need several things． One is warmth． In very cold places almost nothing grows． Plants also need water． In very dry parts of the earth only a few unusual plants can grow． That's why dry deserts everywhere are almost not covered by trees or grass．

Plants must also have a place in which to f，ut down their roots and grow． They find it diffi-culr to grow on hard land． The town is built on hard land． The plants here have only the soil found between the cracks of the stones Io grow in．

Another thing plants must have before they can grow is food．

Whar will happen if we try to make things grow on the sandy beach? A few plants，such as beach grass，will grow in sand，L ut most plants won-t． Even if the weather is warm enough．and we water the plants each day． Many of them will die because the sand on this beach has al-most no food for plants．

( )21． According to the passage，plants need _________ things to grow well．

A． two

B． three

C． four

D． five

A. All OSPF networks require the use of multiple areas

B. Multiple OSPF areas must connect to area 0

C. Single area OSPF networks must be configured in area 1

D. Areas can be assigned any number from 0 to 63535

E. Area 0 is called the backbone area

F. Each OSPF area need to be configured with a loopback interface