function contact_form_prefered(id) {

	id = document.getElementById(id);

    if (id.pref_kontakt.value == "0") {
        id.tel.className = "A_required A_digit";
        id.email.className = "A_email";
    }
    if (id.pref_kontakt.value == "1") {
        id.email.className = "A_required A_email";
        id.tel.className = "A_digit";
    }

}