单项选择题 Most air conditioning units have （） for adjusting the ratio of recirculated fresh airCare must be taken not to reduce the fresh air so that stuffiness or smells arise.

单项选择题 In extreme weather conditions, either hot or cold, control of the air conditioning plant presents （） difficulties.

单项选择题 Because of the reduced （）in the distillation system, some of the sea water boils as it flows through the tubes of the evaporator.