函数y=2x-1的定义域是（ ） A．（12，+∞） B．[12，+∞） C．（-∞，
题型：选择题
问题：
函数y=
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题型：选择题
Everybody hates rats. But in the earthquake capitals of the world – Japan, Los Angeles, Turkey – rats will soon be man’s new best friends.
What happens after an earthquake? We send in rescue dogs. Why? Because they can smell people. Dogs save lives. They help rescuers to find living people. But dogs are big and they can’t get into small spaces. So now a new research project is using a smaller animal to save lives: the rat.
How does it work? First , the rat is trained to smell people. When this happens, the rat’s brain gives a signal（信号）.This is sent to a small radio on its back, and then the rescuers follow the radio signals. When rat’s brain activity jumps, the rescuers know that someone is alive. The rat has smelled that person.
Although there are already robots which can do this job, rats are better. Christian Linster at Cornell University, New York, says, “Robots’ noses don’t work well when there are other smells around. Rats are good at that. ” Rats can also see in the dark. They are cheaper and quicker to train than dogs, and unlike robots, they don’t need electricity!
The “rat project” is not finished, but Julie Ryan of International Rescue Corps in Scotland says, “It would be fantastic. A rat could get into spaces we couldn’t get to, and a rat would get out if it wasn’t safe.” Perhaps for the first time in history, people will be happy to see a rat in a building（but only after an earthquake, of course.）
1．In the world earthquake capitals, rats will bee man’s best friends because they can .
A．take the place of man’s rescue jobs
B．find the position of people alive who are trapped in buildings
C．serve as food for people alive who are trapped in buildings
D．get into small spaces
2．From the third paragraph we know the rescuers can judge a person is alive by .
A．the noise made by the rat
B．the rat’s unusual behaviour
C．the signal sent by the radio on the rat’s back
D．the smell given off by the person
3．In doing rescue jobs, .
A．rats smell better than dogs
B．dogs don’t need to be trained to smell people
C．robots’ sense of smell can be affected by other smells around 
D．rats can see in the dark and smaller than robots
4．Rats have all the following advantages except that .
A．they are more fantastic than other animals
B．they are less expensive to train than dogs
C．they don’t need electricity
D．they are small and can get into small places
5．After reading the passage we can know .
A．at present rats have taken the place of dogs in searching for people
B．the “rat project” has been finished
C．people are now happy to see a rat in a building
D．now people still use dogs and robots in saving people