What does the first period of surD chamber time consist of?

The first period of surD chamber time is characterized by a specific profile designed to manage and mitigate decompression sickness risks. The correct option indicates that the first period consists of 15 minutes at 50 feet of seawater (fsw) and an additional 15 minutes at 40 fsw.

This two-step approach allows the body to off-gas nitrogen effectively while varying the pressure and reducing the risk of developing symptoms related to decompression. Spending an equal amount of time at these two depths is critical because it provides an optimal balance of decompression that promotes safety while aiding in the body's recovery and adaptation to the pressure changes.

In contrast, the other options propose different time distributions or depth combinations that do not align with best practices for managing decompression during the initial phase. By adhering to the specified timing and depths in the correct answer, divers maximize their chances of a safe and effective recovery from potential decompression illness.