Victims of the poisonous gas get sleepier and sleepier until the

Victims of the poisonous gas get sleepier and sleepier until they （）consciousness,never to （）it. 

lose,regain

sink,gain

fail,get

drop,have

毒气的受害者越来越困，直到失去知觉，再也无法恢复。lose失去，regain复得，符合句意。