造成我国野生生物资源减少的主要原因是( )
A. 生存斗争过于激烈
B. 物种退化
C. 自然条件的变化
D. 生存环境的改变和破坏
Whatcausesspeedreductionincylindercushioning?
A. oilflowthroughasmallhole
B. backpressurecreatedinthesystem
C. bothA.andB.
D. noneoftheabove
Whyarehydrauliccylinderscushioned?
A. cushioningdeceleratesthepistonofacylinder
B. stressandvibrationscanbereduced
C. bothA.andB.
D. noneoftheabove
Cylinderendcushioningdecreasespistonspeedbytheendofthepistonmotion.
A. 对
B. 错