var ddlRegions;

$(document).ready(function() {
    ddlRegions = $get("Regions");
    $("select#Countries").change(function() {
        var countryId = $("#Countries > option:selected").attr("value");
        ddlChanged(countryId, ddlRegions);
        });
});

function ddlChanged(countryId, ddl) {
    ddl.options.length = 0;
    ddl.disabled = true;
    $.post("/Callback/RegionsByCountry"
        , { countryId: countryId }
        , function(data) {
            bindRegionResults(ddl, data);
        });
}

function bindRegionResults(ddl, data) {
    var newOption;
    for (var k = 0; k < data.length; k++) {
        ddl.disabled = false;
        newOption = new Option(data[k].Text, data[k].Value);
        ddl.options.add(newOption);
    }
}