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【单选题】
已知某地25岁正常成年男性的平均收缩压为113.0mmHg，从该地随机抽取20名25岁正常成年男性，测得其平均收缩压为119.0mmHg，从同一个地区中再随机抽取20名8岁正常男孩，测的其平均收缩压为90.0mmHg，标准差为9.8mmHg，90.0mmHg与113.0mmHg不同，原因是()。A.
样本例数太少
B.
抽样误差
C.
总体均数不同
D.
系统误差
E.
样本均数不可比
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A.
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B.
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C.
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D.
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