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();