81. Male and female students are quite different from each other ______ the age at which they begin to develop an intellectual self-discipline.
Sandy could do nothing but _________ to his teacher that he was wrong.
--The prison was ___________ fire last night. --Someone must have set fire___________ it.
The __________ is just around the corner and you won't miss it.
__________ she couldn't understand was__________ fewer and fewer students showed interest in her lessons.
Scientists say it may be five or ten years___________ it is possible to test this medicine on human patients.