Questions 27～31 are based on the following passage. The word conservation has a thrifty meaning. To conserve is to save and protect, to leave what we ourselves enjoy in such good condition that others may also share the enjoyment. Our forefathers had no idea that human population would increase faster than the supplies of raw materials; most of them, even until very recently, had the foolish idea that the treasures were 'limitless' and 'inexhaustible'. Most of the citizens of earlier generations knew little or nothing about the complicated and delicate system that runs all through nature, and which means that, as in living body, an un- healthy condition of one part will sooner or later be harmful to all the others. Fifty years ago nature study was not part of the school work; scientific forestry was a new idea; timber was still cheap because it c6uld be brought in any quantity from distant woodlands; soil destruction and fiver floods were not national problems; nobody had yet studied long-term climatic cycles in relation to proper land use; even the Word 'conservation' had nothing of the meaning that it has of us today. For the sake of ourselves and-those who will come after us, we must now set about repairing the mistakes of our forefathers. Conservation should, therefore, be made a part of everyone's daily life. To know about the water table in the ground is just as important to us as a knowledge of the basic arithmetic formulas. We need to know why all watersheds (流域) need the protection of plant life and why the running current of streams and rivers must be made to yield their full benefit to the soil before they finally escape to the sea. We need to be taught the duty of planting trees as well as of cutting them. We need to know the importance of big, mature trees, because living space for most of man's fellow creatures on this planet is figured not only. in square measure of surface but also in cubic volume above the earth. In brief, it should be our goal to restore as much of the original beauty of nature as we can. 第27题：According to the author, the greatest mistake of our forefathers was that_________