Thank you Joffm for your promote reply.
Sorry for the mistake, the array filter is from my attempt of following the example, but it shouldn't be there really. But I don't understand about 'there were no x-axis subquestions, and seems the checkboxes were displayed as the following.
In certain situation actually means in some questions (no equation needed), in other words, sometimes some options are removed.
Thanks again for your time.
Wishing you a merry Christmas and a happy new year!
Regards,
Xueting