arc core

例句：

1. 1.The arc core is the point where the circle touches the ground.
   弧心是圆与地面接触的点。
2. 2. The arc core is the part of the circle that is closest to the center.
   弧心是圆中最靠近中心的那部分。
3. 3. The arc core is the point where the circle is closest to the center.
   弧心是圆与中心最近接触的点。