液压泵按输出油方向能可分为（）。
A.单向泵
B.双向泵
C.定量
D.变量泵
参考答案：A, B
传说中的“神农尝百草”，发明医药，成为中华原始农业和医药业的创始人的是
A．黄帝
B．炎帝
C．舜
D．禹
What is the definition of a Path
A．A sequence of vertices and the edges formed by each successive pair of vertice
B．A walk with distinct vertice
C．A walk with distinct edge
D．None of the above
