I am not convinced. If you put fruit on the table you know there are several things. Fruit is a noun that refers to more than one item. If you want singular, then you have to say a piece of fruit. When you say fruits, it means different types of fruit such as apples and oranges. The same with people. It is possible to say peoples, but in that case it refers to different races or nations of people. Also fish. It is possible to say fishes, but then it means fish of different species. So when I hear the word fruits, it means different kinds of fruit. It can't be all one type such as apples. That is why I can't believe that the sentence "Apples are (a kind of ) fruit" is not correct. How about "Apples are a fruit" ? Is that better? I would appreciate more feedback on this. I know this is not an English course, but why not improve English, too? Thank you.