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【单选题】
Natural flavorings and fragrances are often costly and limited in supply. For example, the vital ingredient in a rose fragrance is extracted from natural rose oil at a cost of thousands of dollars a pound an identical synthetic substance can be made for 1% of this cost. Since the early twentieth century, success in reproducing these substances has created a new industry that today produces hundreds of artificial flavors and fragrances. Some natural fragrances are easily synthesized these include vanillin, the aromatic ingredient in vanilla, and benzaldehyde, the aromatic ingredient in wild cherries. Other fragrances, however, have dozens, even hundreds of components. Only recently has it been possible to separate and identify these ingredients by the use of gas chromatography and spectroscopy. Once the chemical identity is known, it is often possible to synthesize them. Nevertheless, some complex substances, such as the aroma of fresh coffee, have still not been duplicated satisfactorily. Many of the chemical compounds making up these synthetics are identical to those found in nature, and are as harmless or harmful as the natural substances. New products must be tested for safety, and when used in food, must be approved by the U.S. Food and Drug Administration. The availability of synthetic flavors and fragrances has made possible a large variety of products, from inexpensive beverages to perfumed soap to used cars with applied 'new car odor.' From the passage we can learn that ______.
A.
natural flavorings and fragrances are not quite dear
B.
the limitation of natural flavorings and fragrances is clear
C.
the supply of natural flavorings and fragrances is adequate to meet the demand
D.
the cost of producing natural flavorings and fragrances is high
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参考答案:
举一反三
【简答题】将公称尺寸相同的孔、轴安装在一起,孔与轴公差带之间的关系叫做( )
【单选题】当公称尺寸相同的孔的公差带在轴的公差带之上时,孔轴的配合为()
A.
间隙配合
B.
过渡配合
C.
过盈配合
【判断题】( )公称尺寸相同的孔的公差带在轴的公差带之下,孔、轴形成的是间隙配合。
A.
正确
B.
错误
【多选题】在机器装配中,将公称尺寸相同的、相互结合的孔和轴公差带之间的关系称为配合,配合的种类包括
A.
极限配合
B.
过盈配合
C.
间隙配合
D.
过渡配合
【简答题】公称尺寸相同的孔和轴结合时,轴的实际尺寸大于轴的实际尺寸的这类配合称为 ,轴的公差带在孔的公差带的 ,且 。
【多选题】测量误差的来源有哪些?()
A.
人的判断能力
B.
仪器本身的误差
C.
外界环境的影响D测量方法的误差
【多选题】GPS测量误差的来源有哪些?
A.
轨道误差
B.
电离层折射
C.
多路径效应
D.
接收机位置误差
E.
接收机噪声
【判断题】只有公称尺寸相同的孔的公差带和轴的公差带才可以放在同一幅公差带图中。
A.
正确
B.
错误
【单选题】如图所示为公称尺寸相同的孔和轴配合,如果公差带图中,无论孔和轴的尺寸在公差带范围内如何变化,配合结果都如图所示,这种配合属于什么配合。
A.
间隙配合
B.
过渡配合
C.
过盈配合
D.
间隙
【简答题】公称 尺寸相同的 孔和轴结合时,孔的实际尺寸大于轴的实际尺寸的这类配合称为 ;反映在公差带图上,孔的公差带在轴的公差带的 且 。
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