(2009湖南娄底重点高中联考)It is the test system，       the teachers，that is to blame for the heavy burden of middle school students nowadays。

After listening to the teacher’s explanations, John ____ what he had done was wrong. 

He was a(n) ___________singer until the age of 40, when he turned professional.

Tibet is facing a(n)____task to maintain  stability and all government workers must be on their highest guard.

She prefers the sofa made of real leather although it costs more than that made of ______ leather.