刀具夹持部分的定位基面通常有哪些？
参考答案：
平面、外圆柱面、内圆柱面、外圆锥、内圆锥面等五类。
—English is______ too difficult for me. I can't learn it well.
—Don't give up. Nothing is difficult if you work hard. [ ]
A. seldom
B. usually
C. never
D. always
脑血栓形成患者服用阿司匹林的目的是（）
A.防治头晕
B.抗血小板集聚
C.退热
D.扩张血管
E.抗风湿
