把下面的一段话改成被动语态four powerful bombs hir central london during t
把下面的一段话改成被动语态
four powerful bombs hir central london during this morning's rush hour.three exploded on london underground lines and another on a bus.
no one has yet discovered who is responsible for the incidents but police are already collecting evidence.they hope that they can find an explanation for the attacks.
急·`!