我建议只做两个选择，然后更改第二选择基于在第一选择什么。

I would suggest having just two selects, then changing the 2nd select based on what is selected in the first.

$('#typeOfGlass').on('change', function(){
    $('#glassWidth').html('');
    if($('#typeOfGlass').val()==15){
        $('#glassWidth').append('<option value="19">19</option>');
        $('#glassWidth').append('<option value="20">20</option>');
        $('#glassWidth').append('<option value="21">21</option>');
    }else{
        $('#glassWidth').append('<option value="6">6</option>');
        $('#glassWidth').append('<option value="7">7</option>');
        $('#glassWidth').append('<option value="8">8</option>');
        $('#glassWidth').append('<option value="9">9</option>');
    }
});

下面是一个工作示例： https://jsfiddle.net/6hmpa1ax/

Here is a working example: https://jsfiddle.net/6hmpa1ax/