"Lemons and bananas are fruit."
Translation:Les citrons et les bananes sont des fruits.
I think its because all lemons and bananas are fruits not some. "Des" is some
Fruit can be both singular and plural in English. I have a piece of fruit (singular). I like fruit (plural) In French le fruit is singular, les fruits is plural.
The definite article can be used before the subject of a sentence to state general truths about it. This is a general truth about lemons and bananas, but this rule can only apply to subjects, so only lemons and bananas takes a definite article here.
I had «Les citrons et les bananes ...», and then I looked at DuoLingo's suggestion, which had «Des citrons ...». I was a sucker and believed DL would be right, and changed my correct response. I should know better than to trust DL by now. What incentive could they possibly have for trying to mislead the student? :-(