Why does the response to ammā begin with fā in fa-sa-nuyassiruhu?

Supplementary Practice 27 — Comparative Rhetoric of Sūrat al-Shams and Sūrat al-Layl

• To convert the verb to past tense
• To connect the response to the ammā construction with its conditional flavor
• To turn the response into total negation
• To make man a vocative