You're on a roll — Step 1: Recall the formula for relative humidity. Relative humidity ($RH$) is defined as the ratio of the actual vapour pressure to the saturation vapour pressure, expressed as a percentage. $$RH = \frac{\text{Actual Vapour Pressure}}{\text{Saturation Vapour Pressure}} \times 100\%$$ Step 2: Substitute the given values into the formula. Given: Actual Vapour Pressure = $12 \text{ mmHg}$ Saturation Vapour Pressure = $16 \text{ mmHg}$ $$RH = \frac{12 \text{ mmHg}}{16 \text{ mmHg}} \times 100\%$$ $$RH = 0.75 \times 100\%$$ $$RH = 75\%$$ The correct option is C. $$\boxed{\text{C. } 75\%}$$ Dalton's Law of Partial Pressures states that the total pressure exerted by a mixture of non-reacting gases is equal to the sum of the partial pressures of the individual gases. The correct option is B. $$\boxed{\text{B. Sum of partial pressures}}$$ Got more? Send 'em 📸