81. Male and female students are quite different from each other ______ the age at which they begin to develop an intellectual self-discipline.
Attending this course will be of advantage for anyone __________.
Nowadays people sometimes separate their waste to make it easier for it __________.
The chair looks rather hard, but in fact it is very comfortable to __________.
In the Grand Canyon, each rock layer can __________ a chapter in a book.
The children had __________ basketball.