Answer: A) Had been invented.
Explanation for this question is being written.
Answer: B) Would prove.
Answer: A) Had taken.
Answer: B) Had just gone.
Answer: A) Had already boarded.
Answer: A) Had finished.
Answer: B) Had chosen.
Answer: C) Had closed/made.
Answer: A) Past perfect tense.
Answer: A) Had left.
Answer: C) Had got.
Answer: B) Had invited.
Answer: A) Had + past participle.
Answer: A) Had met.
Answer: A) Had gone.
Answer: C) Had
Answer: B) Subject + had + V.3.
Answer: A) Had been signed.
Answer: A) Had fallen.
Answer: C) Had just been.
Answer: B) Past perfect.
Answer: A) Had been.
Answer: D) Had forgotten.
Answer: B) Had completed.
Answer: A) Past
Answer: D) Past Perfect.
Answer: D) Had / seen.
Answer: A) Hadn't realized.
Answer: A) Had studied.
Answer: A) Past Perfect.
Answer: B) They had gone to the park.
Answer: B) Had eaten.
Answer: A) Had come.
Answer: C) Had written.
Answer: C) They have seen the play.
Answer: D) Had finished.
Answer: A) Had already reached.
Answer: B) Had spent.
Answer: C) Had fallen.
Answer: A) Had already explained.
Answer: A) Had departed.
Answer: D) Had had.
Answer: A) J'avais compris.
Answer: C) Had/visited/had gone.
Answer: A) Past perfective.
Answer: D) Had-had left.
Answer: B) Had gone.
Answer: A) Had wanted.
Answer: B) Had hidden.
Answer: C) Past perfect.
Answer: B) Had made.
Answer: D) Had seen.
Answer: A) They had defeated him.
Answer: C) The thieves left before the police arrived.
Answer: C) Had sat.
Answer: A) Had already reserved.
Answer: A) Past perfect.
Answer: A) Had improved.