//绑定调查checkbox选项中的事件
new function() {
    var panel = document.getElementById("diaocha");
    var items = panel.getElementsByTagName("input");
    for (var i in items) {
        items[i].onclick = onCheckingItem;
    }
}
//最多选择三个
function onCheckingItem() {
    if (this.checked) {
        var count = getCheckedCount();
        if (count > 3) {
            this.checked = false;
            alert("对不起，你最多只能选择三项！");
        }
    }
}
//调查选择的数量
function getCheckedCount() {
    var count = 0;
    var items = document.getElementById("diaocha").getElementsByTagName("input");
    for (var i in items) {
        if (items[i].checked) {
            count++;
        }
    }
    return count;
}