下拉菜单上的Javascript警报？

我的表单上有一个javascript警报-我的表单的javascript函数工作正常。除此之外，我还希望javascript在用户没有从两个下拉菜单中进行选择时发出警报。我真的没有太多的javascript经验，所以请记住这一点！这是我目前的代码。我真的很感谢你的帮助

<script type="text/javascript">// <![CDATA[
function validateForm() {
    var a = document.forms["myform"]["inf_field_FirstName"].value;
    var b = document.forms["myform"]["inf_field_Email"].value;
    var c = document.forms["myform"]["inf_field_Phone1"].value;
    var e = document.forms["myform"]["inf_field_City"].value;
    var f = document.forms["myform"]["inf_field_State"].value;
    var g = document.forms["myform"]["inf_field_PostalCode"].value;

    if (a == null || a == "" || a == "First Name Here") {
        alert("Please enter your First Name!");
        return false;
    }
    if (c == null || c == '' || c == "Enter Your Phone Here" || c.length < 9) {
        alert("Please insert your phone number!");
        return false;
    }
if (e == null || e == '' ||e == "City") {
        alert("Please insert your city");
        return false;
    }

    if (f == null || f == '' || f == "State") {
        alert("Please insert your state ");
        return false;
    }

if (g == null || g == '' ||g == "Postal Code" || c.length < 5) {
        alert("Please insert your postal code");
        return false;
    }



    var emailRegEx = /^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}$/i;
    if (document.myform.inf_field_Email.value.search(emailRegEx) == -1) {
        alert("Please enter a valid email address.");
        return false;
    }
}
// ]]>
</script>

//

if(document.getElementById('inf_custom_LevelofInterest').value == '' || document.getElementById('inf_custom_Doyouhavemoneytoinvest').value == ''){
alert('please select ...');
return false;
}