.pcd_ad{display:block;
text-align:center;
max-width:990px;margin:0px auto;
}
.mbd_ad{ display:none}
@media(max-width:768px) {
.pcd_ad{display:none !important;}
.mbd_ad{display:block !important;
text-align:center;
max-width:990px;margin:0px auto;
}
}


<div class="pcd_ad">pc广告</div>
<div class="mbd_ad">wap广告</div>