题目内容：
根据材料,回答题:Flying over a desert area in an airplane, two scientists looked down with trained eyes at trees and bushes. After an hour' s flight, one of the scientists wrote in his book, "Look here for probable metal. " Scientists in another airplane, flying over a mountain area, sent a message to other scientists on the ground, "Gold possible. " Walking across hilly ground, four scientists reported ,"This ground should be searched for metal. " From an airplane over a hilly wasteland a scientist sent back by radio one word: "Uranium. "
None of the scientists had X-ray eyes: they had no magic power of looking down below the earth' s surface. They were merely putting to use one of the newest methods of locating minerals in the ground.., trees and plants as signs that certain minerals may lie beneath the ground on which the trees and plants are growing.
This newest method of searching for minerals is based on the fact that minerals deep in the earth may affect the kind of bushes and trees that grow in the surface.
At Watson Bar Greek, a brook(小溪) six thousand feet high in the mountains of British Columbia, Canada, a mineral search group gathered bags of tree seeds. Boxes were filled with small branches from the trees. Roots were dug and put into boxes. Each bag and box was carefully marked. In a scientific laboratory, the parts of the forest trees were burned to ashes and tested .Each small part was examined to learn whether there were minerals in it.
Study of the roots, branches, and seeds showed no silver. But there were small amounts of gold in the roots and a little less gold in the branches and seeds. The seeds growing nearest to the tree trunks had more gold than those growing on the ends of the branches.
Scientists were flying over a desert or a hilly wasteland or a mountain area in order to search for __________ in the ground. A．gold
B．silver
C．uranium
D．minerals
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