单项选择题
【案例分析题】
Rescue Platform
In the aftermath of the terrorist attacks on the World Trade Center，security experts are trying to develop new ways of rescuing people from buming skysrapers.One idea is a platform.capable of flying vertically and hovering in the air like a helicopter.The platform.would rise up and down alongside a skyscraper and pick up people trapped in high stories.
The idea for the vertical takeoff platform.was hatched more than ten years ago by a Russian aerospace engineer，David Metreveli，who has since moved to Israel.Metreveli’s dedsign，called the Eagle，calls for two jet engines that turn four large horizontal propellers.The spinning of the propellers generates the necessary lift，or upward force，to raise the platform.The more power is supplied to the propellers，the higher the platform.rises.Moving the platform.sideways involves applying differing amounts of power to each propeller.
Helicopters are now used in some cases to get people out of buming buildings，Escape baskets slung from them dangle beside the building for people to climb into.Unfortunately，the baskets cannot reach every floor of a building because the ropes from which they hang become unstable beyond a certain length.
So far，Metreveli has built a small-scale model of the Eagle to test his idea.In the wake of September 11，he has been able to secure enough funding to start building a larger，4-meter by 4-meter prototype，which he calls the Eaglet.
A rescue platform. called the Eagle is capable of moving vertically but not sideways.（）
