Why did God mention the heels as a dual and the elbows as a plural in the verse of ablution?

Dr. Fadil Salih Al-Samarrai answered that every hand has one elbow, while every foot has two ankles. If Allah the Almighty had said (الكعوب - the ankles, plural), it would not have indicated the obligation to wash both ankles, for if they had washed one ankle, it would have sufficed. However, Allah the Almighty intended that each of those addressed should wash up to the ankles.