DS-160 form question

[abcabc01]

My parents are planning to visit on a B2 visa. My father has retired from his job as a Professor but teaches in private coaching classes, where the income is based on how many classes he teaches. On the DS-160 form, there is a question for Monthly Income - is it fair to say it does not apply in this case as it could really vary a lot from one month to another? Greatly appreciate any pointers! Many thanks!

[immihelp]

Average it out for a year.