Thanks for your answer.
I use these methods:
$("<option> bb</option>").attr("value", "1").attr("selected", true).appendTo('#t');
$("<option> bb</option>").attr("value", "1").prop("selected", true).appendTo('#t');
$('#t').val('1');
All of them work in Fire Fox,but my problem is in IE.I can`t see the result in IE.
I don`t know what to do?
Please help me.