Shouldn't the singular version be accepted too?
If you want to make the aswer plural then make the question plural too. The right question should be: Apakah cangkir dan gelas-gelas ini bersih?
you can ask about a cup and a glass. no ini there
"Is this cup and this glass clean" is for "reasons" not accepted (23.9.21)
'the' should be accepted
I think so too