Checking one CheckBox in a ListView checks all of the rest using Flutter

I am completly new to Flutter and Stackoverflow. This is my first question to be in fact so please forgive me if I totaly fail at asking this question. I am trying to make a simple Flutter app that provides a ListView of questions and a checkbox beside each. The user can then choose which question they want to answer. My problem is that when the user checks any of the checkboxes then all get checked and vise versa. The questions themselves are retrieved from a backendless database. The code below is what i have so far. I would really appreciate any help anyone can provide me.

import 'package:flutter/material.dart';

class Questions extends StatefulWidget {
  final List<Map> questionList;

  Questions(this.questionList);

  @override
  _QuestionsState createState() => _QuestionsState();
}

class _QuestionsState extends State<Questions> {
  bool _questionSelected = true;

  Widget _buildQuestionItem(BuildContext context, int index) {
    return ListTile(
      title: Text(widget.questionList[index]['question']),
      trailing: Checkbox(
        value: _questionSelected,
        onChanged: (bool val){
          setState(() {
            _questionSelected = val;
          });
        },
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return ListView.builder(
      padding: EdgeInsets.all(10),
      itemBuilder: _buildQuestionItem,
      itemCount: widget.questionList.length,
    );
  }
}