一种燃料的辛烷等级服从正态分布，其平均等级为98.0，标准差为0.8，现从一批新油中抽出25桶，算得样本均值为97.7。假定标准差与原来一样，问新油的辛烷平均等级是否比原来燃料的平均等级偏低。（α=0.05）
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1 When do people come into your life “for a reason”?
A. When they know you well.
B. When you need them in life.
C. When you can support their career.
D. When they are willing to listen to your troubles.
2 Who will end the friendship with the person that is in your life for a reason?
A. The person himself.
B. You.
C. Neither you nor the person.
D. Both you and the person.
.一个未知参数的无偏估计一定唯一
A. 对
B. 错
设总体X的数学期望为μ,X1,X2,…,Xn为来自X的样本，则X1是μ的无偏估计。
A. 对
B. 错
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