Before selecting the curtain for your newly-decorated house，it’s important to know what colors look great _______ the wall paper

Many air flights _________because of the eruption of the volcano, in _______case, many passengers had no choice but to take a train

When _____ the abacus _____– In the sixth century by the Chinese people.

he world’s population ______ water shortageswithin 15 years according to the World Bank, just one of many recent alarmingreports on the world’s fresh water supply.

You look so sad. What’s wrong with you—I bought ______ bicycle, but when I rode it, one of _____ wheels became flat.