计时器里面的参数time为什么停留在初始值？

js代码如下：

(function() {
                $.fn.countDown = function(userOptions) {
                    var defaultOptions = {
                            duration: 60
                        },
                        opt = $.extend({}, defaultOptions, userOptions),
                        time = opt.duration,
                        clickFlag = true;
                    function cd() {
                        if(time > 0) {
                            $(this).text('剩余' + time + '秒').css('cursor', 'not-allowed');
                            setTimeout(cd, 1000);
                            time--;
                            clickFlag = false;
                        } else if(time == 0) {
                            $(this).text('重新发送验证码').css('cursor','pointer');
                            time = opt.duration;
                            clickFlag = true;
                        }
                    }
                    $(this).on({
                        click: function() {
                            if(clickFlag == true) {
                                cd.call(this);
                            } else if(clickFlag == false) {
                                alert('不能重复点击！');
                            }
                        }
                    });
                };
            }());

            $(document).ready(function() {
                $('#CD_time').countDown({
                    duration: 120
                });
            });
            

经过几个前辈的指点，先修改代码如下：（已能顺利执行）

(function() {
                $.fn.countDown = function(userOptions) {
                    var defaultOptions = {
                            duration: 60
                        },
                        opt = $.extend({}, defaultOptions, userOptions),
                        time = opt.duration,
                        clickFlag = true,
                        _this = this;

                    function cd() {
                        if(time > 0) {
                            $(_this).text('剩余' + time + '秒').css('cursor', 'not-allowed');
                            setTimeout(cd, 1000);
                            time--;
                            clickFlag = false;
                        } else if(time == 0) {
                            $(_this).text('重新发送验证码').css('cursor', 'pointer');
                            time = opt.duration;
                            clickFlag = true;
                        }
                    }
                    $(this).on({
                        click: function() {
                            if (clickFlag == true) {
                                cd.call(this);
                            } else if (clickFlag == false) {
                                alert('不能重复点击！');
                            }
                        }
                    });
                };
            }());
            $(document).ready(function() {
                $('#CD_time').countDown({
                    duration: 120
                });
            });