[单项选择]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 not, such as a ceramic knife or plasti
A. unreliable screening
B. full exposure
C. inadequate efficiency
D. travellers’ modesty

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