阅读理解。
Flying over a desert 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 notebook, "Look here for probable metal." 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 powers for looking down below the earth's
surface. They were merely putting to use one of the newest methods of locating minerals in the ground-using
trees and plants as signs that certain minerals may lie under 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 on the surface.
At Watson Bar Creek, 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 amount of gold in the roots
and a little less gold in the branches and seeds. The seeds growing nearest to the tree trunk had more gold
than those growing on the ends of the branches.
If the trees indicated that there was no gold in the ground, the scientists wouldn't spare money to pay for
digging into the ground.
1. Scientists were flying over a desert or a hilly wasteland or a mountain region to ____.
A. train their eyes
B. study the trees
C. look for gold
D. search for minerals
2. The study of trees, branches and roots indicated that ____.
A. there were larger amounts of gold in the branches far from the tree trunk than in the seeds
B. there were smaller amounts of gold in the tree roots deep in the ground than in the branches
C. there were larger amounts of gold in the seeds growing closer to the tree trunk than in the seeds farther
from it
D. there were larger amounts of gold in the branches than in the leaves on the ends of the branches
3. Which of the following is NOT mentioned as part of a tree that can help the searchers?
A. Leaves.
B. Roots.
C. Branches.
D. Seeds.
4. Which of the following might be the best title for this passage?
A. Scientists searching for treasure with special equipment.
B. New methods of doing geological study.
C. Gold could be found in trees and plants.
D. A new method of searching for minerals.
平传送带上被传送的物体，A为终端皮带轮，如下图所示，A轮半径为r，则m可被平抛出去时，A轮的角速度至少为 。