اية السبب
 

The first line of the metro system opened in 1900
workbook page 33
المفترض was opened

YOU 'RE RIGHT
It's a mistake

الجملة صحيحة لأن يوجد أفعال فى صورة المبنى للمعلوم وتعطى معنى المبنى للمجهول مثل finish / sell

both are right

from "The door was opened" because in the first sentence the doer is not obviously known as it might be wind or human etcWe can say : the door opened in the active voice with a slight difference in meaning

الجملتين صح