如果要实现在多级下拉菜单中选择ul，你可以使用JS来实现。下面是一个示例代码：

HTML:

  Dropdown
  

    

      
Item 1
      
Item 2
      

        Item 3
        

          
Sub-item 1
          
Sub-item 2
          
Sub-item 3
        
      
    
  

CSS:

.dropdown {
  position: relative;
  display: inline-block;
}

.dropdown-content {
  display: none;
  position: absolute;
  background-color: #f9f9f9;
  min-width: 160px;
  box-shadow: 0px 8px 16px 0px rgba(0, 0, 0, 0.2);
  z-index: 1;
}

.dropdown-content ul {
  list-style-type: none;
  padding: 0;
  margin: 0;
}

.dropdown:hover .dropdown-content {
  display: block;
}

.sub-menu {
  display: none;
  position: absolute;
  top: 0;
  left: 100%;
  min-width: 160px;
  z-index: 1;
}

.dropdown-content li.dropdown:hover .sub-menu {
  display: block;
}

JavaScript:

document.addEventListener("DOMContentLoaded", function() {
  var dropdowns = document.getElementsByClassName("dropdown");

  for (var i = 0; i < dropdowns.length; i++) {
    dropdowns[i].addEventListener("click", function() {
      var subMenu = this.getElementsByClassName("sub-menu")[0];
      if (subMenu) {
        subMenu.style.display = (subMenu.style.display === "block") ? "none" : "block";
      }
    });
  }
});

在这个示例中，我们使用了JavaScript来监听每个下拉菜单的点击事件，并根据是否存在子菜单来控制其显示或隐藏。