为什么点击提交，checkTerms(event)没有运行？？（只看没注释的部分）

        <form action="post" method="get">
            <label><input name="" type="radio" value="" id="terms" onclick="chkRadio(this)" />Check to agree to terms &
                conditions</label>
            <p id="termsHint"></p>
            <input type="submit" name="" id="formSignup" value="NEXT" />
        </form>
        <script type="text/javascript">
            var elForm, elSelectPackage, elPackageHint, elTerms;
            var flag = true;

            elForm = document.getElementById('formSignup');

// elSelectPackage = document.getElementById('package');
// elPackageHint = document.getElementById('packageHint');
elTerms = document.getElementById('terms');
elTermsHint = document.getElementById('termsHint');

// function packageHint() {
// var package = this.options[this.selectedIndex].value;
// if (package == 'monthly') {
// elPackageHint.innerHTML = 'Are you sure??';
// } else {
// elPackageHint.innerHTML = 'Wise choice!';
// }
// }
function chkRadio(id) {
id.checked = flag;
flag = !flag;
}

            function checkTerms(event) {
                if (!elTerms.checked) {
                    elTermsHint.innerHTML = 'You must agree to the terms.';
                    event.preventDefault();
                }
            }

            elForm.addEventListener('submit', checkTerms, false);
            elSelectPackage.addEventListener('change', packageHint, false);
        </script>
    </div>
</body>