21. Polar bears live mostly on ____ sea ice, which they use as ____ platform for hunting seals.
A. a, a B. a, the C. /, a D. a, the
22. He hopes to raise _____ about the shortage of clean water on our planet.
A. information B. awareness C. protection D. decision
23. ____ the civil service has its obvious advantages, it doesn’t necessarily suit everyone.
A. While B. Because C. Since D. When