单项选择题 Where an excessive level of contamination is detected an alarm is sounded and diverting valves are operatedThe discharging liquid is then passed to （）.

单项选择题 For the intermediate condition, thermostats must be correctly set （）.

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.