gistfile1.html

<select data-bind="options: tickets, optionsCaption: 'Choose...', optionsText: 'name', value: chosenTicket"></select>
                       
<button data-bind="enable: chosenTicket, click: resetTicket">Clear</button>

<p data-bind="with: chosenTicket">
    You have chosen <b data-bind="text: name"></b>
    ($<span data-bind="text: price"></span>)    
</p>
    
<script type="text/javascript">
    function TicketsViewModel() {
        this.tickets = [
            { name: "Economy", price: 199.95 },
            { name: "Business", price: 449.22 },
            { name: "First Class", price: 1199.99 }
        ];
        this.chosenTicket = ko.observable();
        this.resetTicket = function() { this.chosenTicket(null) }
    }
    ko.applyBindings(new TicketsViewModel(), document.getElementById("liveExample"));
</script>