What is correct about the sākin ṣād in «كَعَصْفٍ»?

Al-Fīl and Quraysh: Applied Iẓhār, Ikhfāʾ, Idghām, Madd and Waqf

• Ṣād is always sīn
• Ṣād is a qalqalah letter
• Ṣād carries separate madd
• Ṣād remains emphatic but receives no qalqalah