How to modify an option in a select list (drop down menu)

How to modify an option in a select list (drop down menu)

My first post here. I can't get my head around modifying a select list so only the item I have appended becomes the selected one i.e. I need to find the one that is already selected and remove 'SELECTED' so it is no longer selected. Here is the outline of my code.......

//Here is the simplified code. (actionOptAdd is activated on the click of a submit button. jSON is used to get the data returned from a PHP function)
//---------------------------------------------------
$(".actionOptAdd").bind("click",
function()
{
.
.
.
$.getJSON('lib/addToProj.php',
{....}, function(data)
}, function(data)
{
$.each(data, function(i,item)
{
if ( i == "project" )project=item;
if ( i == "insertid" )insertid=item;
});
var drp = $('#dropdown');
drp.append("<option value='"+insertid+"'
selected='selected'>"+project+"</option>");
});
}
);
//---------------------------------------
//Thanks in advance.