Yalda is also called 'Shab-e Chelleh' because it opens a traditional forty-day period of what?

• A The harshest cold of winter ✓
• B Ritual fasting
• C Mourning for Siyavash
• D Preparing Nowruz sweets

The 'big chelleh' is the forty coldest days of winter, beginning on the solstice night.