var selectedIndexBeforeChange = 0;

function enableOption(option) {
    option.disabled = false;
    option.style.color = '#000000';
}

function disableOption(option) {
    option.disabled = true;
    option.style.color = '#a0a0a0';
}

function storeSelectedIndexBeforeChange(select) {
    selectedIndexBeforeChange = select.selectedIndex;
}

function unselectDisabledOption(select) {
    if (select.options[select.selectedIndex].disabled) {
        select.selectedIndex = selectedIndexBeforeChange;
    }
}