"The best students read the books."
Translation:Discipuli optimi libros legunt.
"Discipulae optimae libros legunt" should also be accepted (as I have reported.) Yes, Latin has/had a default masculine for mixed-gender or gender-unknown plurals (as here). So do French and Spanish; yet the Duolinguo French and Spanish courses accept feminine answers for all gender ambiguous/unknown situations. Latin should be the same throughout the course.