"this kind of question" or "this kind of questions"

Question

Google Ngram gives this plot

It seems that "this kind of question" is more common. I guess I could deduce that "this kind of question" is more idiomatic than "this kind of questions".

The question is whether both are grammatical?

Answer 1

Both are grammatical, but the expression with plural "questions" seems unnecessary, since a "kind of question" can refer to any number of questions.