Those who have ________ floods and sandstorms know the importance of environmental protection.

The key _____ four modernizationslies _____ the science and education

I feel it hard to keep up with my classmates. But whenever I want to ， my teacher always encourages me to work harder.

How beautiful your hometown is!—Yes, trees and flowers _______ everywhere.

ses, ____were already full, were surrounded byan angry crowd.