In old China, the poor suffered from cold and hunger ________ the rich enjoyed all the good things of life.

A．as

B．when

C．while

D．therefore

一块草地、一片森林、一片沙漠是（）生态系统；土地生态经济系统是（）和复合而成；系统工程=+系统观点+数学方法+计算机技术。