A、A map about health centers. B、An organization on the Internet. C、A piece of computer software. D、A magazine on diseases. C

　　(20) When disaster strikes, Twitter is always the first to know. Now a new study has demonstrated that using Twitter updates and online news websites to track a disease outbreak is not only quicker than more traditional methods—it’s just as reliable, too. In a study published in the January issue of the American Journal of Tropical Medicine and Hygiene, researchers studied the progression of a cholera epidemic in Haiti after the devastating earthquake in 2010. (21)A piece of software called HealthMap was used to monitor how many times the epidemic was mentioned online during the first 100 days of the outbreak. The researchers discovered that 4 697 online reports were released via HealthMap in eight different languages. They found that information spreading from online sources in this way closely matched the official reports, gathered by surveying hospitals and health clinics.   (22) The only difference—and huge advantage—was that the online data was available in almost real time, nearly two weeks before the official reports from the government health ministry were available. "The techniques that were employed eventually could be used around the world as an affordable and efficient way to quickly detect the onset of an epidemic and then intervene with such things as vaccines and antibiotics," says Chunara, who works at the Children’s Hospital in Boston. Twitter data has been used in the past to track vaccinations in the US as well as the progress of diseases such as the swine flu pandemic in 2009 and the annual dengue fever outbreak in Brazil.
　　20. What can Twitter do according to the passage?
　　21. What is HealthMap?
　　22. What’s the advantage of online data over the official reports?