Johns proved his ability as a salesman by______.

参考解析：

细节理解题。根据第二段第五句“You can well imagine that he had increasing business in asbestos roofing materials.”可知，他从事的行业是石棉防火材料，所以A项表述错误；根据第二段第六句“However, because it was very expensive to transport them from Italy to the United States, Mr. Johns sent out a young scientist to seek a source nearer home.”可知，把石棉从意大利运来美国很贵，所以约翰先生就派一名年轻的科学家去附近找源头。所以B项、C项错误；根据第二段第二句“He experimented with the material and then showed is surprising powers to his customers.”可知，他通过用石棉做实验，向他的客户展示出这种材料的神奇之处。所以A项、B项、C项错误。故选D。