建筑学中，拱道或门口的拱心石，是把建筑整体连接在一起来防止它倒塌的石头。
In architecture, a keystone in an archway or doorway is the stone that holds the whole thing together, and keeps it from collapsing.
计划中的大号拱心石石油管道将用于运输产自加拿大油沙的重原油。
The proposed Keystone XL pipeline would carry heavier crude oil produced from tar sands in Canada.
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