单选题
第一篇 Hair Detectives
Scientists have found a way to use hair to figure out where a person is from and where that person has been. The finding could help solve crimes, among other useful applications.
Water is central to the new technique. Our bodies break water down into its parts: hydrogen(氢) and oxygen. Atoms(原子) of these two elements end up in our tissues and hair.
But not all water is the same. Hydrogen and oxygen atoms can vary in how much they weigh. Different forms of a single element are called isotopes(同位素). And depending on where you live, tap water contains unique proportions of the heavier and lighter isotopes of hydrogen and oxygen.
Might hair record these watery quirks(古怪举动；怪癖). That"s what James R. Ehleringer, an environmental scientist at the University of Utah in Salt Lake City, wondered.
To find out, he and his colleagues collected hair from barbers and hair stylists(发型师) in 65 cities in 18 states across the United States. The researchers assumed that the hair they collected came from people who lived in the area.
Even though people drink a lot of bottled water these days, the scientists found that hair overwhelmingly(压倒性地) reflected the concentrations of hydrogen and oxygen isotopes in local tap water. That"s probably because people usually cook their food in the local water. What"s more, most of the other liquids people drink including milk and soft drinks contain large amounts of water that also come from sources within their region.
Scientists already knew how the composition of water varies throughout the country. Ehleringer and colleagues combined that information with their results to predict the composition of hair in people from different regions. One hair sample used in Ehleringer"s study came from a man who had recently moved from Beijing,China, to Salt Lake City. As his hair grew, it reflected his change in location.
The new technique can"t point to exactly where a person is from, because similar types of water appear in different regions that span a broad area. But authorities can now use the information to analyze hair samples from criminals or crime victims and narrow their search for clues(线索).