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重点讲解：
1. impose on 强制实行；强加；
The conditions imposed on volunteers were stringent.
对志愿者的限定条件非常严格。
2. be intended to do sth. 想要；打算；计划；
The defences are intended to obstruct any advance by tanks and other vehicles.
这些防御工事用于阻止坦克等车辆前进。
3. call off 取消；
He has called off the trip.
他已取消了这次行程。
4. threaten to do sth. (坏事)可能发生，将要发生；
The fighting is threatening to turn into full-scale war.
这场战斗有可能会演变为全面的战争。
vt. 扩大，升高，增强
vi. 逐步升级
