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【单选题】
生物体内结合并激活受体的细胞外物质称为第一信使。下列物质中,属于第一信使的是A.
血红蛋白
B.
神经递质受体
C.
生长激素
D.
唾液淀粉酶
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A.
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B.
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B.
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C.
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D.
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A.
is, billions
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