20世纪（）年代，世界玉米产区大面积种植的多为感病品种，致使该病流行成灾。
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学生在空间和时间上结成群体，但成员间尚无共同活动的目的和内容的群体是()。
A. 松散群体
B. 联合群体
C. 正式群体
D. 集体
BTS3812机柜从上至下由开关盒、风扇框、（）、（）、电源框和NLPA框等机框构成。
As to the piston cooling, of the following, which statement is not true?（）
A. Because the piston is exposed to very high temperature, it must be cooled
B. The lower the piston cooling temperature is, the better it is
C. Both water and lube oil can be used as coolant
D. One of the piston cooling methods is vibration cooling, with better effect
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