A statement made and signed by a person on appearance in response to any summons issued under section 70 during the course of any inquiry or proceedings under this Act shall be relevant, for the purpose of proving, in any prosecution for an offence under this Act, the truth of the facts which it contains,––

   (a)  when the person who made the statement is dead or cannot be found, or is incapable of giving evidence, or is kept out of the way by the adverse party, or whose presence cannot be obtained without an amount of delay or expense which, under the circumstances of the case, the court considers unreasonable; or

   (b)  when the person who made the statement is examined as a witness in the case before the court and the court is of the opinion that, having regard to the circumstances of the case, the statement should be admitted in evidence in the interest of justice.