In the workbook that I'm working with, there is a sentence which states as follow:
The animals taken to the slaughterhouse must be killed by humane methods".
According to what I have learnt in the past (see here for example), while using the passive voice (3rd form of the irregular verb) it should be with a 'coefficient' in form of "to be" auxiliary verb - as follow:
The animals that are taken to the slaughterhouse must be killed by humane methods".
Then why the book's sentence omits the auxiliary verb in this case?