Ten years ________a long time in one's life.
As is known to all language learners, newly learnt words will soon be forgotten unless ______ used in everyday communication.
Many students listened to the lecture, but I wondered how much they ______.
I think that something has to be done to stop the air from being polluted, ______ ?
It has been reported that some government leaders ______ their authority and position to get illegal profits for themselves.
Let's learn to use the problem we are facing ______ a stepping-stone to future success.