فَبِأَىِّ ءَالَآءِ رَبِّكُمَا تُكَذِّبَانِ
Fabi ayyi aalaaa'i Rabbikumaa tukazzibaan
Öyleyken, Rabbinizin nimetlerinden hangisini yalanlarsınız?
So which of your Lord's blessings will the two of you deny?
Tukadhdhiban is takdhib, pronouncing a thing false, which is a heavier charge than disbelief, since it assumes the item has already been set out and looked at. Bi-ayyi then asks which one, singular, pushing the hearer to name a particular favour rather than wave the whole set away. Here the item set out is a boundary keeping two waters from swallowing each other.