Código PHP:
<form class="form-horizontal" id="formdependiente">
<label></label>
<select name="ano" class="selectdepend" id="ano">
<option value="">-Seleccionar año-</option>
<?php while($r= mysql_fetch_object($c)){
echo "<option value=".$r->id.">".$r->name."</option>";
} ?>
</select>
</div>
<div class="col-md-6">
<div class="form-group">
<label></label>
<select name="marca" class="selectdepend" id="marca">
<option value="">-Seleccionar marca-</option>
</select >
</div>
</div>
<div class="col-md-6">
<div class="form-group">
<select name="modelo" class="selectdepend" id="modelo">
<option value="">-Seleccionar modelo-</option>
</select>
</div>
</div>
<div class="col-md-6">
<div class="form-group">
<select name="motor" class="selectdepend" id="motor">
<option value="">-Seleccionar motor-</option>
</select>
</div>
</div>