题目

Sustainable development is long-term ________which focuses on the environment and  ________ natural resources．        A．plan; preserve                 B．plan; preserving        C．planning; preserving          D．planning ;preserve 答案：C 解析: 略椭圆C短轴的一个端点与两个焦点F1、F2构成边长为2的正三角形，P为椭圆C上一点，且|PF1|-|PF2|=1，则△PF1F2的面积为    ．