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<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
<script>
$(function ()
{
$('#b_clear').click(function ()
{
document.querySelector('#s_city').options[0].selected = true;
// $('#s_city option:first').prop({'selected' : true});
//$('#s_city option:disabled').prop({'selected' : true})
})
})
</script>
</head>
<body>
<select id="s_city" name="city">
<option disabled="" selected="selected">Выберите Город</option>
<option value="2">Абакан</option>
<option value="3">Барнаул</option>
</select>
<br>
<button id="b_clear">Очистить</button>
</body>
</html>