Read the followingpassage and mark the letter A, B, C or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks.

When man first learnt how to make fire, he began to use fuel for the first time. The fuel he used was probably wood. As time passed, man eventually discovered that substances such as coal and oil would (21)_____.

Coal was not used very widely as a (22)_____ of energy until the last century. With the coming of the industrial revolution, it was soon realized that production would double if coal was used instead of wood. Nowadays, (23)_____ of the huge factories and electricity stations would be unable to function if there was no coal.

In the last twenty or thirty years, (24)_____, the use of coal has declined. As the result, there have been changes in the coal industry. It is (25)_____ that more people would use coal if oil and gas were not available.

man eventually discovered that substances such as coal and oil would (21)_____.