In recent years, the ______ dry heat had led to a series of natural disasters.

 He studies the subject _____. 

As is often pointed out, knowledge is a two-edged weapon which can be used ________ for good or evil．

As is known to all, the newly learnt words will soon be forgotten unless_____ used in everyday communication.

—Do you think our teacher is　　　　？—Yes， he always corrects our homework and prepares his lessons　　　　.