切削平面、基面和主截面三者的关系总是（）的。
A.相互垂直
B.相交
C.相互平行
D.相互倾斜
参考答案：A
In the early 1960s, the famous Swiss psychologist Piaget established his theory of______ development.
唇癌多发生于（）
A.上颌骨
B.下颌骨
C.上唇
D.下唇
E.鼻
