In the early hours of March 8, a Boeing 777 took off from Kuala Lumpur, Malaysia. Its destination was Beijing. But for unknown reasons, it never arrived there. There were 239 people on the Malaysia Airlines flight, including 154 Chinese. As of March 13, 12 different countries, including Malaysia, China, Vietnam and the US, were searching for the plane. The disappearance is an "aviation   mystery", Azharuddin Abdul Rahman, the head of Malaysia's Civil Aviation Authority, said on March 10.There was no clear sign of a crash by March 13. Between 1-2 hours after takeoff, the plane suddenly lost contact with people on the ground. The weather was clear, and the pilots didn't make any calls . No evidence was found in the area where the flight last made contact. People are also talking about a possible hijacking  . International police agency Interpol confirmed on March 9 that at least two passengers on the flight had used stolen passports to get on board. "We are looking at all possibilities," said Malaysian Transport Minister Hishamuddin Hussein. The incident is now being called simply a "plane disappearance". So, what are some possible causes of a plane disappearance? An AP story provided a summary. 1. A failure of the plane's body or its engines. However, even if both engines stopped working, the plane could still glide   for up to 20 minutes, giving pilots time to make an emergency call. 2. Bad weather. Planes are designed to fly through most severe storms. However, in June 2009, an Air France flight from Rio de Janeiro to Paris crashed during a bad storm over the Atlantic Ocean. 3. A bomb. Throughout history, several planes have been brought down by bombs. 4. An accidental shoot-down by some country's military. In July 1988, the US Navy accidentally shot down an Iran Air flight. In September 1983, a Korean Air Lines flight was shot down by a Russian fighter jet. No matter how unlikely a situation, it's too early to determine what really happened to MH370. It could take months, if not years, to _ any possibilities, say experts.. How many cities are mentioned in this passage?
Four cities. five cities. Six cities. Seven cities.