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第六部分：完形填空
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 burning skyscrapers. One _______(51) is a platform capable of flying vertically and hovering in the air _______ (52)a helicopter. The platform would rise up and down alongside a skyscraper and pick up people _______ (53) in high stories.
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