How to recognize a passive sentence?
1) Check if there is a form of 'to be' used.
> am, are, is, was, were, been
2) Is it followed by a participle? (voltooid deelwoord)
3) Is it followed by 'by + noun or pronoun'?
if not, can it be followed by for example; 'by zombies'?
all answers are 'YES!'. That's a passive sentence.