The history of man’s exploration of the earth extends over 5,000 years. The earliest 【B1】______explored in a very limited way: t

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问题     The history of man’s exploration of the earth extends over 5,000 years. The earliest 【B1】______explored in a very limited way: they had to 【B2】______to gather plants and hunt animals to feed themselves. Also, in order to feel more【B3】______in their homes, they had to 【B4】______their surroundings quite thoroughly. But the wider world remained a mystery to them. It was only later, once the necessities of life had been obtained, that people began to wonder what lay 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【B5】______unknown lands and seas.
    The explorers of today are the astronauts and their【B6】______is space, but this does not mean there is nothing left to explore on earth. 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【B7】______. Enormous parts of other continents remain practically unknown. And yet all these places are nowadays【B8】______easy to reach. There is little difference in attitude between the men【B9】______space in our time and the explorers of an earlier age.
    The【B10】______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 to travel where no one has travelled before, even at the risk of his life.
【B5】

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答案previously

解析 分析句子结构可知,句子中不缺少主要成分,故本空应填一副词,修饰形容词unknown。previously意为“先前,以前”。
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