问题：
[单选题]． 1.光滑面对物体的约束反力，作用在接触点处，其方向沿接触面的公法线()。
A、指向受力物体，为压力
B、指向受力物体，为拉力
C、背离受力物体，为拉力
D、背离受力物体，为压力
参考答案：
A
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