当页面内容的高度不满手机屏幕时，footer Div会消失，请指点
当页面内容的高度不满手机屏幕时，footer Div会消失，并且不知道怎么解决，请各位大神指点。
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>二维码</title>
<link type="text/css" rel="stylesheet" href="../iquan/css/style.css?t=2" />
<meta name="viewport" content="width=device-width">
</head>
<body style="position:relative">
<div class="wrapper">
<div class="header_title">
<a href="#">
<div class="header_title_btn"><img src="/iquan/images/fengcai/fanhui.png" /></div>
</a>
<div class="header_title_text_groupon">二维码</div>
</div>
<div class="acticle">
<div class="QR_Code_content">
<p>夏日装扮大酬宾<br><span>加菲猫美甲店</span></p>
<img src="/iquan/images/groupon/QR-Code.png">
</div>
<!--<div class="bgcolor2"></div>-->
</div>
<div class="footer_activity_groupon_t" >
<a href="#"><div class="footer_QR_Code">
<img src="/iquan/images/groupon/pic_download.png" >
</div></a>
</div>
<div class="footer_activity_place_holder"></div>
</div>
</body>
</html>
-----------------css----------------------!
.footer_activity_groupon_t{
max-width:1080px;
width:1080px;
margin:auto 0px 0px 0px;
height:141px;
padding:0px;
position:fixed;
bottom:0;
color:#575757;}
