The correct answer is: B. Increase, decrease.
The dew point is the temperature at which the air is saturated with water vapor. The relative humidity is the ratio of the amount of water vapor in the air to the amount of water vapor that the air can hold at that temperature.
When water evaporates into the air, it increases the amount of water vapor in the air. This increases the dew point. However, it does not increase the relative humidity. This is because the relative humidity is a ratio, and the denominator (the amount of water vapor that the air can hold at that temperature) remains the same.
Therefore, if the air temperature remains constant, evaporating water into the air will increase the dew point and decrease the relative humidity.
Here is a diagram that may help to illustrate the concept:
[Diagram of a graph with two lines, one for the dew point and one for the relative humidity. The dew point line is increasing, while the relative humidity line is decreasing.]
The dew point line is increasing because the amount of water vapor in the air is increasing. The relative humidity line is decreasing because the amount of water vapor in the air is increasing, but the temperature is remaining constant. This means that the air is becoming more saturated with water vapor, but it is not becoming any warmer.