Add options for sources:
<select id="sources"></select>
function showSources(values) {
var sources = document.getElementById('sources');
var option;
var count = values.length;
if (count == 0) {
option = document.createElement('option');
option.text = "N/A";
sources.appendChild(option);
}
else {
for (var i = 0; i < count; i++) {
option = document.createElement('option');
option.text = values[i];
sources.appendChild(option);
};
}
}