1. **Problem:** Marie descends to 15 feet below sea level and Nancy descends to 22 feet below sea level. Find the difference in their depths. 2. **Formula:** Difference in depths = Depth of Nancy - Depth of Marie 3. **Calculation:** $$\text{Difference} = (-22) - (-15) = -22 + 15 = -7$$ 4. **Explanation:** Since both depths are below sea level, they are negative numbers. Subtracting a negative is the same as adding its positive. 5. **Answer:** The difference in their depths is $-7$ feet, which corresponds to option c).