级联下拉列表不会从数据库加载(MVC，JSON，C#)

<select id="parentSelect" onchange="loadChildOptions()">
  <option value="">请选择上级选项</option>
  <option value="1">选项1</option>
  <option value="2">选项2</option>
  <option value="3">选项3</option>
</select>

<select id="childSelect">
  <option value="">请选择下级选项</option>
</select>

<script>
function loadChildOptions() {
  var parentId = document.getElementById("parentSelect").value;
  if (parentId !== "") {
    // 发送请求获取下级选项数据
    var xhr = new XMLHttpRequest();
    xhr.open("GET", "/api/getChildOptions?parentId=" + parentId, true);
    xhr.onreadystatechange = function() {
      if (xhr.readyState === 4 && xhr.status === 200) {
        var childOptions = JSON.parse(xhr.responseText);
        // 清空下级选项
        var childSelect = document.getElementById("childSelect");
        childSelect.innerHTML = "<option value=''>请选择下级选项</option>";
        // 动态添加下级选项
        for (var i = 0; i < childOptions.length; i++) {
          var option = document.createElement("option");
          option.value = childOptions[i].value;
          option.innerText = childOptions[i].text;
          childSelect.appendChild(option);
        }
      }
    };
    xhr.send();
  } else {
    // 清空下级选项
    var childSelect = document.getElementById("childSelect");
    childSelect.innerHTML = "<option value=''>请选择下级选项</option>";
  }
}
</script>

public class ApiController : Controller
{
    public ActionResult GetChildOptions(string parentId)
    {
        // 根据parentId从数据库中查询下级选项数据
        List<Option> childOptions = GetChildOptionsFromDatabase(parentId);
        // 将下级选项数据封装成JSON格式返回给前端
        return Json(childOptions, JsonRequestBehavior.AllowGet);
    }

    private List<Option> GetChildOptionsFromDatabase(string parentId)
    {
        // 从数据库中查询下级选项数据的逻辑
        // ...
    }
}

public class Option
{
    public string Value { get; set; }
    public string Text { get; set; }
}