ANS:B - decrease

If air (a non-condensing gas) is present in a condensing vapor stream, it will decrease the condensation rate of vapor. Condensation occurs when a vapor changes phase to a liquid upon contact with a cooler surface. The presence of air in the condensing vapor stream creates a barrier between the vapor and the condensing surface. This barrier inhibits the direct contact between the vapor molecules and the condensing surface, reducing the efficiency of the condensation process. As a result, the condensation rate decreases. So, the correct option is: decrease.