单项选择题
The single acting cylinder is the simplest form where pressure is applied to the full area of the piston head to move the load through piston rod extensionThe load causes rod （） when the drive pressure is released.
A.contraction B.retraction C.expansion D.shrinkage
单项选择题 Among the basic categories of equipment, which of the following are used to convert the hydraulic energy into linear motion?（）
单项选择题 In a general cargo crane, （） is fitted to raise or lower the jib.
单项选择题 External and internal leakage in hydraulic systems may be caused by （）.
