He didn't _____ what I read because his mind was on something else.
With a good __________ of Chinese, Helen was hired to do the task of communication with the Chinese company.
The teacher commanded that the students __________ the classroom before he returned.
Before the tourists set off, they spent much time setting a limit __________ the expenses of the trip.
There is a limit __________ people's life, but no limit __________ the knowledge.
Unfortunately, the research was limited __________ increasing economic profits,__________ protecting the environment.