Why is this impersonal se?

Since guardar is a transitive verb, I assumed this would be passive...perhaps I am missing something? Or is this an error on SD?
Thanks for the explanations!
3 Answers
Oh, also it uses "guardan". I thought with impersonal, only third person singular was allowed?
Sunsmile, "guardar" is plural because "los libros" is plural. The sentence is passive voice.
Annie has given you an example of impersonal, which is not really a case but just a way to depersonalize a phrase by using 3rd person. To do this, we use only 3rd person plural. "Me han robado" ..... "They have robbed me."
Oh, also it uses "guardan". I thought with impersonal, only third person singular was allowed?

Sunsmile, "guardar" is plural because "los libros" is plural. The sentence is passive voice.
So it appears there is an error in the quiz answers.