html5如何使用canvas画空心圆与实心圆

<canvas id="canvas" width="500" height="500" style="background-color: yellow;"></canvas>

var canvas=document.getElementById("canvas");     var cxt=canvas.getContext("2d");     //画一个空心圆     cxt.beginPath();     cxt.arc(200,200,50,0,360,false);     cxt.lineWidth=5;     cxt.strokeStyle="green";     cxt.stroke();//画空心圆     cxt.closePath();     //画一个实心圆     cxt.beginPath();     cxt.arc(200,100,50,0,360,false);     cxt.fillStyle="red";//填充颜色,默认是黑色     cxt.fill();//画实心圆     cxt.closePath();     //空心和实心的组合     cxt.beginPath();     cxt.arc(300,300,50,0,360,false);     cxt.fillStyle="red";     cxt.fill();     cxt.strokeStyle="green";     cxt.stroke();     cxt.closePath();