Although man has known asbestos for many hundreds of years, it was not until 160 years ago that it was mined for the first time on the North American continent. H. W. Johns, owner of a New York City Supply Shop for roofers, was responsible for the opening of that first mine.
Mr. Johns was given a piece of asbestos which had been found in Italy. He experimented with the material and then showed its surprising powers to his customers. After putting on a pair of asbestos gloves, which looked much like ordinary work gloves, he took red-hot coals from the fireplace and played with them in his hands. How astonished the customers were to discover that he was not burned at all. You can well imagine that he had increasing business in asbestos roofing materials. However, because it was very expensive to transport (carry) them from Italy to the United States, Mr. Johns sent out a young scientist to seek a source nearer home. This young man found great vein, in the province of Quebec in Canada.
Ever since 1881 Quebec has led the world in the production of this unusual mineral, which is made up of magnesium, silicon, iron, and oxygen. When it is mined, the asbestos is heavy, just as you would expect a mineral to be. When it is separated, a strange thing happens: the rock breaks down into fine, soft, soapy fibres.
Scientists do not know why the rock can be separated easily into threads, but they have found thousands of uses of this fireproof material, of the so-called "cloth of stone".
The author’s main purpose in writing this passage was to（）
A. show the need for more scientists
B. compare asbestos with other minerals
C. increase the sales of asbestos
D. present facts about asbestos
