问题
单项选择题 A1/A2型题
颈椎牵引治疗时，开始牵引重量一般控制在（）左右
A.10kg
B.15kg
C.5kg
D.3kg
答案
参考答案：C
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颈椎牵引治疗时，开始牵引重量一般控制在（）左右
A.10kg
B.15kg
C.5kg
D.3kg
参考答案：C
| 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." |
______, a substance can be synthesized.
A．Upon identifying the basic components of it
B．Once chemically analyzed
C．When gas chromatography is used
D．If spectroscopy is adopted