问题补充：
We were told not to touch the equipment in the laboratory unless .
A. allowed to do
B. allowing to do
C. allowed to
D. allowing to
答案：
A. allowed to do
B. allowing to do
(答案→)C. allowed to
D. allowing to
解析：考查省略，后面的句子省略了we were…touch the equipment in the laboratory，句子意思“我们被告诉不能碰实验室的设备除非我们被允许碰”，所以选C
