如何修剪表单'；提交'；多级选择列表的值

我有一个带有多级选择列表的表单，选择列表显示基于先前选择的选项（使用jQuery）。你可以在我的帖子或这里找到小提琴。我的表单操作如下：

<form action="catalog.php?category=submit" method="GET" id="selectorForm">

单击提交按钮时返回的URL如下：

.../catalog.php?series=5series&3generation=&5generation=e39&e46model=&e39model=525i&e60model=

但是，我只希望在URL中传递多级选择列表中选择的最后一个选项。例如：../catalog.php?category=525i。

我该怎么做？

或者，如果我获得当前catalog.php的URL，我如何根据php中选择列表的级别来确定传递了哪些级别值？

jQuery(function($) {
  $("#series").change(function() {
    var targetID = $(this).val();
    $("div.generation").hide();
    $('#' + targetID).show();
  });
  $("#3generation").change(function() {
    var targetID = $(this).val();
    $("div.model").hide();
    $('#' + targetID).show();
  });
  $("#5generation").change(function() {
    var targetID = $(this).val();
    $("div.model").hide();
    $('#' + targetID).show();
  });
  $('.selectModel').change(function() {
    if ($(this).val() != "") {
      $('#submit').show();
    } else {
      $('#submit').hide();
    }
  });
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<h3 align="center">Choose your BMW<br /><br /></h3>
<div id="selectorFormContainer" width="100%" align="center">
  <form action="catalog.php?category=submit" method="GET" id="selectorForm">
    <div id="SeriesContainer" class="selector">
      <select id="series" class=" validate['required']" name="series" size="1">
        <option value="">Choose your BMW's series</option>
        <option value="3series">3 series</option>
        <option value="5series">5 series</option>
      </select>
    </div>
    <br />
    <div id="3series" class="generation" style="display: none;" onchange="ChangeDropdowns(this.value)">
      <select id="3generation" class="selectGen" name="3generation" size="1">
        <option value="">Choose your BMW's generation</option>
        <option value="e46">E46 (1998 - 2006)</option>
      </select>
    </div>
    <div id="5series" class="generation" style="display: none;" onchange="ChangeDropdowns(this.value)">
      <select id="5generation" class="selectGen" name="5generation" size="1">
        <option value="">Choose your BMW's generation</option>
        <option value="e39">E39 (1995 - 2003)</option>
        <option value="e60">E60 (2003 - 2010)</option>
      </select>
    </div>
    <br />
    <div id="e46" class="model" style="display: none;" onchange="ChangeDropdowns(this.value)">
      <select id="e46model" class="selectModel" name="e46model">
        <option value="">Choose your BMW's model</option>
        <option value="320i">320i</option>
      </select>
    </div>
    <div id="e39" class="model" style="display: none;" onchange="ChangeDropdowns(this.value)">
      <select id="e39model" class="selectModel" name="e39model">
        <option value="">Choose your BMW's model</option>
        <option value="525i">525i</option>
        <option value="540i">540i</option>
      </select>
    </div>
    <div id="e60" class="model" style="display: none;" onchange="ChangeDropdowns(this.value)">
      <select id="e60model" class="selectModel" name="e60model">
        <option value="">Choose your BMW's model</option>
        <option value="M5">M5</option>
      </select>
    </div>
    <br />
    <div id="submit" style="display: none;">
      <input value="submit" style="border: none; border-radius: 5px; height: 2em; width: 10em; background-color: #22b8f0; color: #ffffff;" type="submit" />
    </div>
  </form>
</div>