加急!-------THEY HAD TAKEN OFF THEIR SHOSE WERE THEY ALLOWED T

选D Not until
Not until they had taken off their shoes were they allowed to get into the lab.
意思是：他们不把鞋子脱掉的话是不被允许进入实验室的.
因为后面是用的倒装“were they...”故前面应是否定倒装“not until”
原句等价于：If they had taken off their shoes,they were allowed to get into the lab.
not...until=if