问题
配伍题 B1型题
人胚8～9个月主要造血器官是（）人胚第3～9周主要造血器官是（）胚胎第6周，肝脏造血发生，其造血干细胞主要来源于（）
A.肝
B.脾
C.淋巴结
D.骨髓
E.卵黄囊血岛
答案
参考答案：D,E,A
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人胚8～9个月主要造血器官是（）人胚第3～9周主要造血器官是（）胚胎第6周，肝脏造血发生，其造血干细胞主要来源于（）
A.肝
B.脾
C.淋巴结
D.骨髓
E.卵黄囊血岛
参考答案：D,E,A
Have you ever wondered why there are so many skin colors in the world? Do you know why people living in particular areas usually have a certain color? Biology and history are the two reasons for this. Skin contains something called melanin, which determines a person’s skin color. The more melanin a person has, the darker his or her skin will be. The amount of and the production of melanin are controlled by genetics, but can be affected by other things, such as sunlight. If a person lives in a place with less sunlight, a person’s body will produce less melanin, making the skin lighter. Skin color is also affected by another source ---- vitamin D. humans all need vitamin D to build bones. People can get it by eating foods such as fish and milk, or from sunlight, so sunlight absorbed by melanin cannot be used for vitamin D production. Therefore, a dark—skinned person will produce less vitamin D than a light—skinned person when they received the same amount of sunlight. The connection between vitamin D production and skin color is clear when we look at evolution. The earliest humans lived in Africa, their dark skin produced less vitamin D because of their dark skin. As a result, their skin made less melanin, so they could get enough sunlight to produce vitamin D. their skin gradually got lighter and they lost hair. Now, people living in areas with strong sunlight like Africa, have darker skin, while people living in other areas have lighter skin. The exception to this is the Inuit, who live in a place with little sunlight, but have dark skin because they eat a lot of fish and have enough vitamin D. Evolution has given us a rainbow of skin colors. Humans have always had melanin to determine our skin color. What has changed through history is the environment where we have lived. This has in turn changed our melanin production, and eventually, skin color.
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