He ________ your car by mistake because he didn't have the keys.
A．mustn't have taken B．shouldn't have taken
C．couldn't have taken D．needn't have taken
—The similarities between social networks are striking.
—That ________ come as no surprise，because they are all products of the Internet.
A．should B．shall C．may D．must
—Hi，Joan.Any idea where Tom is?
—He ________ on the playground.I saw him there playing football just now.
A．shall be B．should have been
C．must be D．might have been
—Guess what？I came first in that contest.
—Great！You ________ a lot during the past two weeks.
A．must practise B．should practise
C．should have practised D．must have practised
—Where will you start your work after graduation?
—Mum，it's not been decided yet.I ________ continue my study for a higher degree.
A．need B．must C．would D．might
Many people think of the brain as a mystery.They don't know much about intelligence and how it works.When they do think about what intelligence is，many people believe that a person is born smart，average，or dumb—and stays that way in the whole life.But new research shows that the brain is more like a muscle—it changes and gets stronger when you use it.And scientists have been able to show just how the brain grows and gets stronger when you learn.
Everyone knows that when you lift weights，your muscles get bigger and you get stronger.A person who can't lift 20 pounds when he/she starts exercising can get strong enough to lift 100 pounds after working out for a long time.That's because the muscles become larger and stronger with exercise.And when you stop exercising，the muscles shrink and you get weaker.That's why people say “Use it or lose it！”
But most people don't know that when they practice and learn new things，parts of their brain change and get larger a lot like muscles do when they exercise. Inside the cortex(皮层)of the brain are billions of tiny nerve cells，called neurons. The nerve cells have branches connecting them to other cells in a complicated network. Communication between these brain cells is what allows us to think and solve problems. When you learn new things，these tiny connections in the brain actually multiply and get stronger.The more you challenge your mind to learn，the more your brain cells grow.Then，things that you once found very hard or even impossible to do—like speaking a foreign language or doing algebra(代数)—seem to become easy after learning them for a period of time.The result is a stronger，smarter brain.
Scientists started thinking that the human brain could develop and change when they studied animals'brains.They found out that animals that lived in a challenging environment were more perspicacious—they were better at solving problems and learning new things.
1.According to the first paragraph，________.
A．the function of our brain is like that of the muscle
B．until now it's impossible to explain the brain's mystery
C．many people believe one's intelligence is naturally determined
D．one's brain grows stronger as the age increases
2.Training muscles is compared to ________.
A．using the brain
B．connecting things in your brain
C．lifting weights
D．doing research about the brain
3.What does the underlined word“perspicacious”in the last paragraph probably mean?
A．Strong. B．Smart.
C．Popular. D．Active.
4.The following paragraph will most probably talk about ________.
A．the differences between animals'brains and humans'brains
B．the relation between human brains and muscles
C．scientists' findings about animals' brains
D．how to make your brain smarter through selfdevelopment
