21. As a matter of fact, space flight is ________ high risk job and _______exploration of the moon is particularly so.

A. /; an       B. a; /          C. a; the        D. the; an

22. ---Come on! Please give me some ideas about the project.
---Sorry. With so much work ______________my mind, I almost break down.
A. filled           B. filling C. to fill              D. being filled

23. The school rules state that no child shall be allowed out of the school during the day, ______ accompanied by an adult.