今天我们要学的词是 leapfrog. leapfrog 原指儿童玩的青蛙跳游戏，引申下来就是跳升，越级。The U.S. government said it was working to "harden" the border crossing at Tijuana, Mexico, to prepare for the arrival of a migrant caravan leapfrogging its way across western Mexico. 美国政府表示，正在努力“加强”位于墨西哥蒂华纳的过境点，准备应对越过墨西哥西部的移民大军到来。The U.S. is on pace to leapfrog both Saudi Arabia and Russia to become the world's biggest oil producer. 美国正在逐步超越沙特阿拉伯和俄罗斯，成为世界上最大的石油生产国。