set a default selection on select box dropdown list using jquery.

set a default selection on select box dropdown list using jquery.

Here this is my for following code for creating dependent select box using jquery.
what i am exactly needed is how to set default selection of dropdown,I am struggle on this selection.Please help me guys... 
  1.   
  2. var categories = {
  3.           "Select":[{value:'select',text:'Select'}],

  5. "United States":[{value:'0', text:'Select'},{value:'55', text:'Currency'},{value:'67', text:'Card'},{value:'56', text:'Cheque'}],
  6.           "European":[{value:'0', text:'Select'},{value:'30', text:'Currency'},{value:'40', text:'Card'},{value:'50', text:'Cheque'}],
  7.           "Britian":[{value:'0', text:'Select'},{value:'10', text:'Currency'},{value:'20', text:' Card'},{value:'30, text:'Cheque'}],
  8.           "Australia":[{value:'0', text:'Select'},{value:'1', text:'Currency Notes'},{value:'2', text:'Card'},{value:'3', text:'Cheque'}],
  9.              };
  11. function selectchange($this) {
  12.               var select = $this.parents('.product-item').find('.orderbox2');
  13.               select.empty();
  14.               $.each(categories[$(':selected', $this).text()], function () {
  15.                   select.append('<option value="' + this.value + '">' + this.text + '</option>');
  16.               });
  17.           }

  19. $(function () {
  20.           $("#product").on("change", '.orderbox1', function() { selectchange($(this)) });
  21.           $("#product").append(categories);
  22. });
  23. </script>
  24. <body>
  25. <DIV class="product-item>
  26. <div>

  28. <select  name="item_currency[]" id="orderbox1" class="orderbox1" >
  29.                   <option value="">Select</option>
  30. <option value="usa">United states</option>

  32. <option value="gbp">Britian</option>

  34. <option value="eur">European</option>
  35. </div
  36.  <div>

  38. <select name="item_size[]" id="orderbox2" class="orderbox2" >
                       </select>
</div>
</div>