function rollover(id) {
    document.getElementById("b_" + id).src = 'images/btn_' + id + '_s.png';
    document.getElementById("c_" + id).className = 'selected';
}

function rollout(id) {
    document.getElementById("b_" + id).src = 'images/btn_' + id + '_n.png';
    document.getElementById("c_" + id).className = '';
}