单项选择题

抽样的基本原则是（）

A．随机化原则
B．标准化原则
C．标准化原则
D．标准化原则

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单项选择题

若气门间隙过大时，则气门开启量
A、不变
B、变小
C、变大
D、不一定

单项选择题

Part Three
Directions: In this section, you will hear four short conversations and two long conversations. At the end of each conversation, you will hear four questions. Both the conversation and the questions will be spoken only once. After you hear a question, you must choose the best answer from the four choices marked A, B, C and D.

Long conversation 1
17. What is the new challenge in the second stage of the experiment?
A、There are more levers to press.
B、Dolphins are to respond to more signals.
C、Dolphins are put in a bigger tank.
D、Only Doris could see the lights.