问题
选择题
“生理盲点”是指视网膜上连接（ ）的部位，因为没有感光细胞所致．
A．玻璃体
B．晶状体
C．视神经
D．脉络膜
答案
答案：C
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“生理盲点”是指视网膜上连接（ ）的部位，因为没有感光细胞所致．
A．玻璃体
B．晶状体
C．视神经
D．脉络膜
答案：C
| 选择句子补全对话。 | |
Sarah: She is at home. Teacher: ___________________________________________________________________________ Sarah: She is sick. She has a cold. Teacher: ___________________________________________________________________________ Sarah: Yes, she has a headache. Teacher: ___________________________________________________________________________ Sarah: Yes, she did. And the doctor asked her to take some medicine and stay in bed for one day. Teacher: Oh, ________________. I hope she will be better soon. |
| 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