二、单项选择（每小题0.5分，20小题，共10分）
11. Usually children are allowed to ______ when they are six years old.
A. attend school B. attend the school C. join school D. join the school
12. Yang Liwei was surrounded by the audience ______ he stepped off the stage.
A. until B. through C. now that D. immediately
13. ______ many times, but he still couldn’t understand it.
A. Having been told B. Though had been told
C. He was told D. Having told
