题目内容：
根据下面内容，回答题： It takes a long time to grow a tree．How long? Well, pine trees are the quickest growing trees, but still, they take twenty years to reach a size suitable for cutting and harvesting．An oak takes about sixty years to grow to a good size．A redwood may take hundreds of years．Lumber companies, which make their money on trees, depend on those that grow quickly．Therefore, they are always looking for methods to make trees grow faster．So far, the secret to fast growing trees seems to he in " super-seeds．" These are seeds that are gathered from the quickest growing trees in a forest．One company searched 100,000 acres of trees and selected the seeds from just fifteen trees.These two ounces of seed were enough to plant several hundred new trees．Eventually, when these trees have grown, seeds will again be taken only from the fastest growing trees of crop．This process of artificial selection will yield, in the future, a super-tree that will grow in half the time it takes normal trees to develop．Unfortunately, it takes a long time for a plan like this to reach its goal．The Weyerhauser Lumber Corporation, which started its first collection of seed in 1958, is just now beginning to harvest the super-seeds of the first generation of the faster growing trees．
This selection centers on________． A．the life span of trees
B．the age of redwoods
C．making trees grow quickly
D．cutting trees for lumber
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