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  3. Until the nineteen thirties, most Christmas trees grew wild in the forest or in abandoned fields; Early tree farming took place

Until the nineteen thirties, most Christmas trees grew wild in the forest or in abandoned fields; Early tree farming took place

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问题     Until the nineteen thirties, most Christmas trees grew wild in the forest or in abandoned fields; Early tree farming took place mostly on poor and marginal soils where some conifers could still thrive.
(41)______
    On a well-managed Christmas tree farm, ninety percent of the land is occupied by vigorously growing tree crops at all times. Grass grows between the rows of trees. With an average tree production cycle of ten years, counted from the time a seedling is planted, this ensures the farmer a regular annual crop ready for harvest.
(42)______A broad network of Tools holds the soil and a continuous ground cover prevents surface erosion by water and wind. As a long-term crop, trees allow a natural buildup of bird and animal populations. Tree farms provide stable refuge and feeding areas for wildlife, often very near large urban centres, at no cost to the taxpayer. Some farms allow visits at different times of the year. Their accessibility and the quiet, park-like surroundings make these farms very pleasing to people. Like all plants,the conifers growing for eventual harvest as Christmas trees remove carbon dioxide from the atmosphere as they grow and produce oxygen as a byproduct. One acre of Christmas trees produces enough oxygen for 18 people. Without this process of photosynthesis, life could not exist on earth.
(43) ______The hardest work is in the spring and summer when tree fanners must control competition from other plants. Smaller conifers are especially vulnerable to being crowded out by weeds which compete for space and moisture. Late spring and summer is also the time when insects and disease can take hold to the point where serious damage can occur. A tree farmer must keep watch for this and react in time to save the crop from serious damage.
(44)______There are also large and small farms which supply stores, garden centres and tree lot operators with Christmas trees. On these farms, Christmas trees ready for harvest are often marked in advance with a tag or tape. They are then cut and each tree is baled (打包, 捆包) using a machine which presses the branches against the trunk, holding them in place with netting or twine.
(45)______But it is a risky business, because if there are no trees to sell one year, there is also no income, while all the family’ s expenses and the expense of operating the farm and caring for the trees that are still growing. Christmas tree farm families know that the REAL tree product grown on their farm symbolizes the REAL spirit of Christmas; the faith and the hope of harmony among all the world’s people.
[A] The busiest time of year for Christmas tree farmers is the short period when trees must be harvested and sold. Farmers near towns and cities’ often invite families to their farms to choose and cut their own perfect Christmas tree.
[B] Christmas tree conifer seedlings are grown from seed in beds or greenhouses. Except on the largest tree farm operations, this is usually a separate business, requiring a lot of skill and attention. [C] Their basic production characteristics make Christmas trees an extremely friendly farm crop, both to our environment and to people and animals.
[D] Today almost all Christmas trees are cultivated as a specialty crop on farms suited for their production. Tree farming has become an important agricultural activity that provides work for thousands of Canadians every year.
[E] Looks are very important in determining how much a conifer is worth as a Christmas tree. Too much or not enough rain, sun, or shade and insects, disease, rodents, hail, fire, vandals or trespassing snowmobilers can all render a tree unsaleable.
[F] Some people may think that growing trees in Ontario for harvest at Christmas time is easy, but growers know that it is a full-time year-around job.
[G] Growing Christmas trees, like other types of farming is a very rewarding way to make a living. Christmas tree farming is both a business and a job for the farmer and his family.
选项
答案C
解析 根据下文内容，从段落一致性方面来判断。通读下文可知，这段主要在讲树木对环境的作用，如保持土壤、防止地表水蚀和风蚀等；对动物及人类的好处，如有利于鸟类和动物数量的增加，为野生动物提供稳定的避难所及摄食区，为人们提供一年几次的游览、提供充足的氧气等。由此可知，此处选项应该为[C]，即本段的主题句，意为：它们基本的生长特点使得圣诞树成为对环境、人和动物都极其有利的农场作物。
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