填空题

The history of man’s exploration of the earth extends over 5,000 years. The earliest cavemen (36) in a very limited way: they had to go out to (37) plants and hunt animals to feed themselves. Also, in order to feel more (38) in their homes, they had to investigate their (39) quite thoroughly. But the wider world remained a (40) to them. It was only later, once the (41) of life had been obtained, that people began to wonder what (42) on the other side of the hill, whether the forest around them ever came to an end, or where the sun went after it had fallen into the sea. This curiosity caused them to discover (43) unknown lands and seas. The explorers of today are the astronauts and their territory is space, (44) . It is true that with the invention this century of jet aircraft and spaceships, every part of the world has been photographed and maps made of it. But there is still much left to find out. (45) . And yet all these places are nowadays comparatively easy to reach. There is little difference in attitude between the men sent into space in our time and the explorers of an earlier age. The universe is as mysterious and exciting for them as our planet seemed to the people in the past. Man’s natural curiosity drives him on to explore the unknown and (46) .