By the middle of the 21st century , the vast majority of the world’s population ________ in cities rather than in the country.
—You were not listening;what troubled you? —I my coming math exam.
I am amazed at someone so rich _____ difficulty in paying what he owes you.
It doesn’t matter how much loss we have suffered; just avoid _____ the mistake again.
Just as sound travels through the air in waves, electricity travels through the air in ______ radio waves.
It was _____ of Jack to turn off the radio when I was sleeping in the bedroom.