Which part identifies the dual in atharaynā?

Imruʾ al-Qays: A Guarded Tent, the Pleiades, Objection, and Dunes (Verses 23–29)

• The oblique dual yāʾ and loss of the dual nūn in the construct
• A final emphatic nūn
• An initial interrogative hamzah
• A medial purpose-lām