题目

Sustainable development is applied to just about everything from energy to clean water and economic growth, and as a result it has become difficult to question either the basic assumptions behind it or the way the concept is put to use. This is especially true in agriculture, where sustainable development is often taken as the measure of progress without a proper appreciation of historical and cultural perspectives.To start with, it is important to remember that the nature of agriculture has changed markedly throughout history, and will continue to do so. Medieval agriculture in northe6．宇宙中两颗相距较近的天体成为“双星”，它们以两者连线上的某点为圆心做匀速圆周运动，而不至于因万有引力的作用吸到一起，设二者的质量分别为m1和m2，下列说法正确的是（　　）A．双星的轨道半径和质量成正比关系B．双星的线速度和质量成反比关系C．双星的向心加速度和质量成反比关系D．双星所受的力和质量成反比关系