CaCO3与PbI2的溶度积非常接近,均为约1×10-9,下列叙述中正确的是()。A.二者的饱和溶液中Ca2+、pb2CaCO3与PbI2的溶度积非常接近,均为约1×10-9,下列叙述中正确的是()。
A. 二者的饱和溶液中Ca2+、pb2+离子浓度接近
B. 溶液中含Ca2+与Pb2+离子的浓度相同时,逐渐加入浓度均为0.1 mol/dm3的Na2CO3和NaI的混合溶液,首先沉淀的是CaCO3
C. 二者的饱和溶液中,Ca2+离子的浓度为pb2+离子的浓度的1/2
D. 二者在水中的溶解度近似相等
The expression "snag" (Line 3, Paragraph 2) most probably means ______.
A. trap.
B. obstacle.
C. criticism.
D. advisory.
Part A
Directions: Read the following four texts. Answer the questions below each text by choosing A, B, C or D. (40 points)
When Melissa Mahan and her husband visited the Netherlands, they felt imprisoned by their tour bus. It forced them to see the city according to a particular route and specific schedule—but going off on their own meant missing out on the information provided by the guide. On their return home to San Diego, California, they started a new company called Tour Coupes. Now, when tourists in San Diego rent one of their small, brightly coloured three-wheeled vehicles, they are treated to a narration over the stereo system about the places they pass, triggered by Global Positioning System (GPS) satellite technology.
This is just one example of how GPS is being used to provide new services to tourists. "What we really have here is a technology that allows people to forget about the technology", says Jim Carrier of IntelliTours, a GPS tourism firm which began offering a similar service over a year ago in Montgomery, Alabama. The city is packed with sites associated with two important chapters in American history, the civil war of the 1860s and the civil-rights movement a century later. Montgomery has a 120-year-old trolley system, called the Lightning Route, which circulates around the downtown area and is mainly used by tourists. On the Lightning Route trolleys, GPS-triggered audio clips point out historical hotspots.
Other firms, such as CityShow in New York and GPS Tours Canada in Banff, Canada, offer hand-held GPS receivers that play audio clips for listening to while walking or driving. In South Africa, Europcar, a car-rental firm, offers a device called the Xplorer. As well as providing commentary on 2,000 points of interest, it can also warn drivers if they exceed the local speed limit.
If such services prove popular, the use of dedicated audio-guide devices could give way to a different approach. A growing number of mobile phones have built-in GPS or can determine their locations using other technologies. Information for tourists delivered via phones could be updated in real time and could contain advertisements. "Location-based services", such as the ability to call up a list of nearby banks or pizzerias, have been talked about for years but have never taken off. But aiming such services at tourists makes sense—since people are more likely to want information when in an unfamiliar place. It could give mobile roaming a whole new meaning.
In the opening paragraph, the author introduces his topic by ______.
A. posing an illustration.
B. justifying an assumption.
C. making a comparison.
D. explaining a phenomenon.