21. It is advised by health experts that windows should be opened now and then for air to _____,which makes the air in your room fresher.

A. go through         B. get through         C. circulate          D. spread around

22. The widespread of super-hybrid rice around the world ______the lives of millions and millions of people suffering from hunger to a great level.

A. improved          B. is improving        C. has improved   D. has been improving