阅读理解
Some families in America and elsewhere have started buying child friendly mobile phones outfitted with GPS (Global Positioning System) technology. These phones and their related tracking services allow parents to pinpoint the location of their children with ease. Parents agree to pick up the phone bill in return for the reassurance of knowing where their children are; children areprepared to put up with the watching if they are allowed to have a phone. Mobile operators in America are now launching tracking services. Under a federal decree known as E911, they had to upgrade their networks to ensure that anyone dialing the 911 emergency number could be located to within 100 metres. Some operators opted for triangulation technology, which determines the location of the handset by comparing the signals received by different base stations. But Verizon and Sprint chose to adopt the more expensive but more accurate GPS technology instead, and are now looking for ways to make money from it. Verizon calls its service "Chaperone". For $10 a month, parents can call up the location of their child's LG Migo handset from their own mobile phones, or from a PC.The child receives a message saying that the handset's position has been requested, and the parents receive an address, or a marker on a web-based map, giving the child's location. For an extra $10 per month, they can sign up for Child Zone, a service that, among other things, fires off an alert when a youngster (or, at least, the youngster's handset) strays outside a specified area For its part, Sprint has launched a similar service that can also let parents know when a child arrives at a particular location. Another location service is available from Nextel, a mobile operator that was taken over by Sprint in 2005.Nextel opened up some of its systems to enable other firms to build their own software and services on top of its GPS technology. One example is AccuTracking, a small company which offers a tracking service for $6 a month and boasts that it is "ideal for vehicle tracking" or to keep "virtual eyes on kids". Some customers are also using the service to track their spouses, by hiding phones in their cars. "Mine is hidden under the hood, hot-wired to the battery—it works very well and it is easy to hook up continuous power, " writes one customer on AccuTracking's message board. Start-ups are working on everything from city-wide games of hide-and-seek to monitoring the locations of Alzheimer's patients. Services that monitor jogging routes, and work out distance travelled and calories consumed, might also prove popular. As a result, mobile operators, handset-makers and start-ups could transform and expand a small, specialist market so far dominated by expensive, dedicated tracking systems.
单选题21.The tracking function of the mobile networks was probably first intended for _____.