Section B
Directions: There are 2 passages in this section．Each passage is followed by some questions or unfinished statements．For each of them there are four choices marked A, B, C and D．You should decide on the best choice.
Noses have their advantages．They're cute, they're versatile, and they're small enough to be carried around．Which may be why the artificial odor detectors that engineers have been building since the 1950's to try to mimic the olfactory(嗅觉的) abilities of our built-in sensors have taken so long to find their way to the market--and then, in most cases, have flopped(失败).
But things are finally looking up for the electronic nose．Thanks to advances in chip technology and pattern-recognition techniques, increasingly tiny sniffers (嗅探器) are beginning to live up to their moniker (模仿者)- Today e-noses are being tested for everything from disease detection to disaster prion, and lower-prices models are starting to come to the market including an $ 8,000 device called the Cyranose 320 being introduced this week by Cyrano Sciences of Pasadena, Calif.
Like our proboscises(长鼻子), e-noses are only as good as their sensors, and all of them operate on oiliness(油质)--rather than to the molecule itself．Working together, the receptors can generate unique principles remarkably similar to those of a real nose．Humans detect odors with up to 650 types of receptors found on cells high up in the nasal passages, somewhere between our eyebrows．How the nose works is still something of a mystery, but it is believed that each receptor responds to a subtle characteristic of a molecule that carries odor--its peculiar shape, say, or degree of 'smell prints' of a wide variety of odors, which are then parceled off to the brain and stored.
In e-noses, chemical sensors replace the body's cellular receptors, and microprocessors substitute for the brain．'What limits these devices is how well the sensors are doing,' explains Nathan Lewis, the Caltech chemist who helped invent the sensor technology licensed by Cyrano and who has since continued his research independently．He compares the power of e-noses to the resolution of computer monitors: 'Are you seeing the world in eight shades of gray or in 16 million colors?'
It can be learned from the passage that ______.