摩天大楼中一部高层的客运电梯，电梯的简化模型如图1所示．电梯的加速度a是随时间t变化的，已知电梯在t=0时由静止开始上升，a─t图象如图2所示．电梯总质量m=2.0×103kg．忽略一切阻力，重力加速度g取10m/s2．
World Wildlife Fund ____(感激) very much the efforts that China has made to protect endangered species from being further endangered.
把下列词语补充完整。
汹涌____ 心惊____ 忐忑____
