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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 identit3, 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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举一反三
【单选题】教师角色特征中的核心特征是( )。
A.
建设者
B.
开发者
C.
促进者
D.
研究者
【判断题】教师即学生学习的促进者,是教师角色特征中的核心特征。( )
A.
正确
B.
错误
【单选题】题号:4796 难度:中 第1章 下列各m的值中,能使m%3==2&&m%5==3&&m%7==2为真的是().
A.
8
B.
23
C.
17
D.
6
【单选题】下列对《祝福》选段的理解与分析,不恰当的一项是 ( ) 冬至的祭祖时节,她做得更出力,看四婶装好祭品,和阿牛将桌子抬到堂屋中央,她便坦然的 去拿酒杯和筷子。 “你放着罢,祥林嫂!”四婶慌忙大声说。 她像是受了炮烙似的缩手,脸色同时变作灰黑,也不再去取烛台,只是失神的站着。直到四叔 上香的时候,教她走开,她才走开。这一回她的变化非常大,第二天,不但眼睛窈陷下去,连精神也更不济了。而且很胆怯,不独怕暗...
A.
“坦然”一词写出了祥林嫂自以为摆脱厄运、免遭歧视的心理。
B.
四婶的断喝运用倒装句式,命令的意味更强烈,态度也更严厉。
C.
祥林嫂前后的变化,说明她对生活感到绝望,精神已经崩溃了。
D.
由选文可见,祥林嫂的悲剧正在于她胆子太小,易受别人影响。
【简答题】影响酶催化反应的因素有: 、 、 、 和 。
【简答题】影响酶催化反应的因素主要有______、______、______、______、______、______和______。
【判断题】中介体: 又称中间体,是细胞膜内陷折叠而成的管状、囊状结构。 其功能类似真核细胞的线粒体,也参与细菌分裂。
A.
正确
B.
错误
【简答题】教师即学生学习的促进者,是教师角色特征中的核心特征。( )
【判断题】format命令可以影响计算精度
A.
正确
B.
错误
【简答题】影响酶催化反应的因素有: 、 、 、 、 、 。
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