题目

What kinds of homes will we live in the future? Nobody can be sure, but architects and scientists are working out new ideas now.Some architects are thinking about building whole cities under huge glass domes（圆顶）. Of course, complicated（复杂的） heating and cooling systems will be necessary to control the climate in the domes. However, there will never be any rain or snow, and the temperature will always be comfortable.Another idea that will be helpful to small countries (and island countries) is the floating city. Monaco has already built homes, stores, and offices on the water o 经过测算，地球的平均半径为（         ） A．6357千米       B．6378千米             C．6371千米       D．40000千米