Which statement keeps the singular and number rules separate correctly?

The Five Nouns: Core Case Forms

• all five forms are singular case endings
• أَبُو/أَبَا/أَبِي are singular case forms; أَبَوَانِ/أَبَوَيْنِ are dual
• أَبُو is dual and أَبَوَانِ is singular
• dual forms never change ending