抽油泵的活塞是由无缝钢管制成空心圆柱体，其外表面镀铬，并带有环状槽沟，环状槽沟的主要作用是（）。
A、放蜡
B、防砂
C、防气
D、防水
参考答案：B
Recent studies have shown that most US graduates receive cash as gifts, though the amount
received ______ from person to person. [ ]
A. prefers
B. promotes
C. varies
D. suffers
根据政党的组织特点可以将政党划分为哪几种类型？
