We often think of future．We often wonder what the world would be like in a hundred years' time. Think of space, perhaps a permanent(永久的) station on the moon will have been set up．Perhaps people will be able to visit the moon as tourists．Cheap rockets for space travel will have been developed, permitting long journeys throughout the solar system(太阳系)．When that time comes, people will be taking holidays in space and visiting other planets．Great progress will have been discovered for the most terrible of all diseases—cancer．Pollution is a problem we must solve．In a hundred years' time it will have been controlled. All the world will have been developed—even Antarctica．There will be large cities in Antarctica．We already have supersonic(超音速的) flight, but in a hundred years' time we will have supersonic land travel as well. We will have used up most of earth's land to build our cities, so floating cities will have been built．The Japanese already have plans for cities of this kind．And there will be cities under the sea．The first of these will have been completed. 39．We will have developed __________ in a hundred years' time, permitting long journeys all over the solar system. A．cheap rockets for space travel       B．a plane for space travel C．a permanent spaceship                D．a satellite for tourists 40．The last paragraph mainly talks about _____________. A．new ways of building cities         B．the building of cities under the sea C．the building of floating cities in Japan D．the er which earth's land will face 41．The best title for the passage is ______________. A．Travel in the Future             B．Future Life C．Where to Live in the Future  D．Space Travel