I have questions regarding the use of "kind of."
I've read on englishplus website that when using kind of always use a singular object of the preposition of. If the object is plural, then use kinds of.
Examples: What kind of salad is this?
(Kind and salad are both singular.)
However, when I browsed eslgold.com, one of the examples showed "What kind of seats does it have?"
I'm bewildered that should the example use "kinds of" in this case according to the rule?
Please advise and explain further the proper use of "kind of".
Thanks very much.