Despite increased airport security since September 11th, 2001, the technology to scan both passengers and baggage for weapons and bombs remains largely unchanged. Travellers walk through metal detectors and carry-on bags pass through x-ray machines that superimpose colour-coded highlights, but do little else. Checked-in luggage is screened by "computed tomography", which peers inside a suitcase rather like a CAT scan of a brain. These systems can alert an operator to something suspicious, but they cannot tell what it is.
More sophisticated screening technologies are emerging, albeit slowly. There are three main approaches: enhanced x-rays to spot hidden objects, sensor technology to sniff dangerous chemicals, and radio frequencies that can identify liquids and solids. A number of manufacturers are using "reflective" or "backscatter" x-rays that can be calibrated to see objects through clothing. They can spot things that a metal detector may n
A. A generalization is made in paragraph 2 and then elaborated in paragraph 3,4 and 5
B. More sophisticated screening technologies are mentioned in paragraph 2 and 3 and then examples are provided in paragraph 4 and 5
C. Specific evidence is provided in paragraph 1,2 and 3 and then a conclusion is drawn in paragraph 4
D. Three main approaches are advanced in paragraph 2,3 and then their functions are detailed in paragraph 4 and 5
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