________ deep down in the earth for thousands of years, the dead forest has rotted away and become coal．

Mary is often heard ________ playing piano in the next room in the evening.

Taiyuan，a city in North China，was struck by a heavy snowfall last week，________serious traffic problems.

Not recognizing me at first, __________.

The Turners consider ________a computer, which is commonly considered _____a great help in our work and study.