jQuery 获取遍历获取table中每一个tr中的第一个td的方法

<tbody id="already_question_list"> 
  <tr> 
    <td><?php echo $val['unique_number']; ?></td> 
    <td><?php echo $val['year'].'-'.$val['series']; ?></td> 
    <td><?php echo $val['content'] ?></td> 
    <td><?php echo $val['knowledges'] ?></td> 
    <td><?php echo $val['last_admin']; ?></td> 
    <td> 
      <input type="button" class="btn" value="－" data-score="<?php echo $val['score']; ?>" onclick='remove_selected(this);'/>\ 
      <input type="hidden" name="question_numbers[]" value="<?php echo $val['unique_number']; ?>"/> 
    </td> 
  </tr> 
</tbody> 
 
<script> 
  var leng = $("#already_question_list tr").length; 
  var filter_numbs = new Array(); 
  for(var i=0; i<=leng; i++) 
  { 
    numberStr = $("#already_question_list tr").eq(i).find("td:first").html(); 
    filter_numbs.push(numberStr); 
  } 
</script>