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>