How to enable and disable drop down list by selecting radio button in ASP.NET

I want to create a page in which whenever i select a particular radio item, a drop down list should be enabled and if some other item it selected the drop down list should be disabled.

This is my form script in ASP.NET

<form id="ticketbooking" class="book-ticket" runat="server">
    <asp:Label ID="mode" runat="server" class="label">Mode of Travel</asp:Label><br />
    <asp:RadioButton name="selector" ID="train" GroupName="travelmode" runat="server" value="tr"></asp:RadioButton>
    <asp:Label ID="forTrain" for="train" runat="server" class="label">Train</asp:Label> 
    <div class="check"></div>
    <asp:RadioButton name="selector" ID="air" GroupName="travelmode" runat="server" value="ar" OnCheckedChanged="air_CheckedChanged"></asp:RadioButton>
    <asp:Label ID="forAir" for="air" runat="server" class="label">Air</asp:Label> 
    <div class="check"></div><br />
    <asp:Label ID="airname" runat="server" class="label">Airline Name</asp:Label><br />
    <asp:DropDownList class="droplist" ID="airlist" runat="server" Enabled="false" Font-Size="20px">
        <asp:ListItem>Air India</asp:ListItem>
        <asp:ListItem>Kingfisher</asp:ListItem>
        <asp:ListItem>Jet Airways</asp:ListItem>
        <asp:ListItem>Spice Jet</asp:ListItem>
    </asp:DropDownList>
</form>

I have tried a few scripts as below but they wont work...

Method 1:

<script>
    $(document).ready(function () {
        $('input[name="selector"]').click(function () {
            if ($('input[name="selector"]').is(':checked')) {
                var radioValue = $("input[name='selector']:checked").val();
                if (radioValue == "tr") {
                    $("#airlist").prop("disabled", true);
                } else if (radioValue == "ar") {
                    $("#airlist").prop("disabled", false);
                }
            }
        });
    });
</script>

Method 2:

protected void air_CheckedChanged(object sender, EventArgs e)
{
    if (air.Checked == true)
    {
        airlist.Enabled = true;
    }
    else
    {
        airlist.Enabled = false;
    }
}

I tried both but its not working..when i click on the "Air" radio button the drop down list should be enabled..am i doing something wrong? need some suggestions please..i am still new to ASP.NET and not sure about how to work in this framework.