Past Simple vs Past Perfect

Hello,

Could you tell me if you agree with the correct answers of this test?



Q1 - By the time we ____, everyone had left.

arrived
had arrived



Q2 - I ___ a long time ago.

finished
had finshed
Either could be used here.



Q3 - By that time, everybody ____ exhausted.

was
had been
Either could be used here. Why not 'had been exhausted?'



Q4 - She told me she ____ it ages ago. Can either be used here?

did
had done
Either could be used here.



Q5 - I ____ of it until you mentioned it.

didn't hear
hadn't heard Can either be used here?



Q6 - I ____ enough time to finish the exam.

didn't have
hadn't had
Either could be used here. Can either be used here too?



Q7 - If I ____, I'd have helped.

knew
had known
Either could be used here.



Q8 - If I ____, I'd tell you.

knew
had known
Either could be used here.



Q9 - No sooner ____ than I realised what was going on.

did I arrive
had I arrived
Either could be used here.



Q10 - Until last month, I ___ it before.

didn't try
hadn't tried


Thank you.