Read the article below and decide which answer best fits each space. Đọc đoạn văn dưới và chọn từ thích hợp cho mỗi chỗ trống.

When man first learned how to make a fire, he began to use fuel for the first time. The first fuel he used was probably wood. As time ___(1)___ man eventually discovered that substances such as coal and oil ___(2)___. Coal ___(3)___ very widely as a source of energy until the last century. With the coming of the industrial revolution, it was soon realised that production ___(4)___ if coal was used instead of wood. Nowadays, many of the huge factories and electricity generating stations ___(5)___ to function if there was no coal. In the last twenty or thirty years, however, the use of coal ___(6)___. As a result, there have been changes in the coal industry. It ___(7)___ that more people ___(8)___ coal if oil and gas were not so readily available. There is more than enough coal in the world for man's needs for the next two hundred years if our use of coal ___(9)___ .Unfortunately, however, about half of the world's coal ___(10)___. Mining much of it would be very expensive even if it was possible to use new equipment.