题目内容

以下选项不属于概率抽样方法的是

A. 单纯随机抽样
B. 系统抽样
C. 分层抽样
D. 目的抽样
E. 配额抽样

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抽样的原则是：保证样本的、选取有的样本。

[音频]3. A) Thefts of snowmobile dogs in Alaska.B) A series of injuries to snowmobile drivers.C) Attacks on some Iditarod Race competitors.D) A serious accident in the Alaska sports event.4. A) He stayed behind to look after his injured dogs.B) He has won the Alaska Iditarod Race four times.C) He received a minor injury in the Iditarod Race.D) He has quit the competition in Alaska for good.

带速为30m/s，传递功率为250kW，要求传动比准确，可选用（）。

A. 平带
B. V带
C. 多楔带
D. 同步齿形带

带传动的张紧力为F0，紧边的拉力为F1，松边的拉力为F2，该带传动的有效拉力为（）。

A. F1-F0
B. F2-F0
C. F1-F2
D. F2-F1