The problem with people in Africa ____that the more is given and the more ______.

By the time he was twelve, Edison ________ to make a living by himself .

—Are you a teacher?—I ________, but now I am working in a company.

Your desk is covered with too many unnecessary things, including used papers and books. You’d better ________ some of them. 

23. The writing brushes ____bamboo and animal’s hair.