google上的地图如何实现放大缩小的
<script src="http://ditu.google.cn/maps?file=api&v=2&key=ABQIAAAAg95Bs2zJzg2q9O1Fc3SS7BRGNujmFMQRTMcFTIA7WEub23XdcRQub0CgiHWD0bo_ZWQfKyMuVxwkDw&sensor=true_or_false"
type="text/javascript"></script>
<script type="text/javascript">
function initialize() {
if (GBrowserIsCompatible()) {
var map = new GMap2(document.getElementById("map_canvas"));
map.setCenter(new GLatLng(31.3, 120.6), 13);
}
}
</script>
</head>
<body onload="initialize()" onunload="GUnload()">>
<form id="form1" runat="server">
<div>
<div id="map_canvas" style="width: 500px; height: 300px"></div>
</div>
</form>
</body>
这样实现了加载地图,但是我想让它可以根据我的鼠标滚动来控制地图的放大缩小