"However they are seldom."
Translation:Sie sind jedoch selten.
In English you need to add a verb or an adjective after seldom (In your example, it would be "they are seldom dangerous", for example). Seldom has to qualify something or it means nothing. The message I think you are writing is "However they are rare."
my answer was "jedoch sie sind selten". duolingo doesn't accept it. why is it incorrect?