Try this out…
$('select option:first-child').attr("selected", "selected");
Another option would be this, but it will only work for one drop down list at a time as coded below:
var myDDL = $('myID');
myDDL[0].selectedIndex = 0;
Take a look at this post on how to set based on value, its interesting but won’t help you for this specific issue:
Change the selected value of a drop-down list with jQuery