关于js中按钮类型是image时表单验证问题---点击一次可以验证再次点击直接提交--的问题

function _submit(){
var tableForm = document.getElementById("jvForm");
tableForm.action="${base}/coursequestion/inner_super/o_update.jspx";
tableForm.onsubmit=null;
tableForm.submit();
}
function _check(){
var reg = new RegExp("^[A-D]+$");
var answer = document.getElementById("answer").value;
if(!reg.test(answer)){
alert("答案格式不对！");
return false;
}
_submit();
return true;
}
<form id="jvForm" method="post" onsubmit="return _submit();">
<input type="image" src="/${res}/images/submit.gif" value="submit" onclick="return _check()"/>