如何使用HTML5画布找出点击圈子的哪个部分？

I am trying to create Simon game using HTML5 canvas and vanilla JavaScript. I am confused about the coordinate system in the arc() method. I have divided the circle into 4 quadrants and would like to alert the number of the quadrant clicked. But, I am not sure how to find out which part of the circle was clicked. I am trying to create Simon game using HTML5 ca