無法填充組合框選項

Question

$i = 1; 
$tbl = ""; 
$field = ""; 

if($_POST['jenis']=="Bahan Baku") 
{ 
    $tbl = "bahanbaku"; 
    $field = "bb_"; 
} 
else if($_POST['jenis']=="Bahan Penunjang") 
{ 
    $tbl = "bahanpenunjang"; 
    $field = "bp_"; 
} 

<select class="span10" name="jenis" class="add-on"> 
    <?php 
    $opt = "SELECT bb_nama FROM ".$tbl.""; 
    $result = mysql_query($opt); 
    if($row = mysql_fetch_array($result)) 
    { 
     echo "<option>". $row[$field.'nama'] ."</option>"; 
    } 
    ?> 
</select> 

結果是沒有顯示選定的行。哪一行是錯誤的？