there is a contradiction between what my teacher says and what is written in my book.He says that 3.5 is high voltage but the book says that every gate has its own range for high and low.This 3.5 value was just an example in the book where it was falling in the category of high voltage may be that's why my teacher said it so but i don't think how would we answer this question?

Q:- which of the following represents HIGH voltage?
d)can't be predicted

This was a question in my test and i answered it (d) because i thought it's different for different gates so can't be predicted but my teacher just wronged it saying that it should be (a)...........!
please clear my confusion!