Extjs中grid初始化时选中指定行的问题

<link rel="stylesheet" type="text/css" href="extjs/resources/css/ext-all.css" />
<script type="text/javascript" src="extjs/adapter/ext/ext-base.js"></script>
<script type="text/javascript" src="extjs/ext-all.js"></script>
<script>
var aData = [['11','12'],['21','22'],['31','32'],['41','42']];

Ext.onReady(function(){

    var oRecord = Ext.data.Record.create([
        {name: 'f1', type: 'string'},
        {name: 'f2', type: 'string'}
    ]);

    var store = new Ext.data.Store({
        proxy: new Ext.data.MemoryProxy(aData),
        reader: new Ext.data.ArrayReader({},oRecord)
    });
    store.load();
    var sm = new Ext.grid.CheckboxSelectionModel({singleSelect:false});
    var columns = new Ext.grid.ColumnModel([
        new Ext.grid.RowNumberer(),
        sm,
        {header: '字段1', dataIndex: 'f1'},
        {header: '字段2', dataIndex: 'f2'}
    ]);

    grid = new Ext.grid.GridPanel({
        title: '列表',
        autoHeight:true,
        region: 'center',
        loadMask: true,
        store: store,
        cm: columns,
        sm: sm,
        renderTo:'pad'
    });

    setTimeout(function(){
         grid.selModel.selectRows([0,2]);
    },300); 
});

</script>
<div id='pad'></div>