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【多选题】
保证使用压缩空气启动装置的柴油机迅速而可靠的启动必须具备的条件是( )。A.
供气要有一定定时和适当延续时间
B.
使柴油机由静止到转动的外界能源
C.
保证柴油机能够启动的最少气缸数
D.
启动装置中必须有两个启动空气瓶
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【简答题】一个函数的图象关于y轴成轴对称图形时，称该函数为偶函数，那么在下列四个函数①y=2x；②y=-3x-1；③ ；④ 中，偶函数是（ ）（填出所有偶函数的序号）。
【单选题】Which of the following statements is TRUE?
A.
The subjects in a cross-sectional research are not of the same age group.
B.
The methods of study in longitudinal research will not change over time.
C.
Longitudinal research is reliable only in seeking understandings of the effects of early experiences on later development.
D.
Cross-sectional methods are not usually adopted in studying, for example, the changes in intelligence over a thirty-year period.
【单选题】The optimum cross-sectional thickness for an elastomeric impression materials is( )
A.
0．5mm
B.
1．0mm
C.
2．0mm
D.
4．0mm
【单选题】Which of the following is NOT a characteristic of a cross-sectional study?
A.
It is conducted at only one point in time.
B.
It can be exploratory, descriptive, or explanatory.
C.
It concentrates on the changes that take place among a specific sample over a period of time.
D.
It provides a means for studying a large population at the same point in time.
E.
All of these choices are characteristics of the cross-sectional study.
【简答题】成本按经济用途可以分为( )和( )两在类。
【简答题】成本按经济用途可以分为哪几类？
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