ANS:C - high operating cost.

For absorbers, a high pressure drop typically results in decreased efficiency. A high pressure drop means that the gas must overcome greater resistance as it passes through the absorber. This increased resistance often leads to higher energy consumption for pumping or compressing the gas, which can result in higher operating costs. Additionally, a high pressure drop can lead to operational issues such as uneven distribution of the gas or liquid phases within the absorber, which can negatively impact efficiency. While a high pressure drop may improve gas-liquid contact to some extent by enhancing mixing and increasing interfacial area, the associated drawbacks usually outweigh this potential benefit, leading to decreased overall efficiency in absorber systems.