[单项选择] Human beings have used tools for a very long time. In some parts of the world you can still find tools that people used more than two million years ago. They made these tools by hitting one stone against another. In this way they broke off pieces from one of the stones. These chips of stone were usually sharp on one side. People used them for cutting meat and skin from dead animals, and also for making other tools out of wood. Human beings needed to use tools because they did not have sharp teeth like other meat-eating animals, such as lions and tigers. Tools helped people to get food more easily. Working with tools also helped to develop human intelligence. The human brain grew bigger, and human beings began to invent more and more tools and machines. The stone chip was one of the first tools that people used, and perhaps it is the most important. Some scientists say that it was the key to the success of mankind. Since 1960 a new kind of tool has appeared. This is the si
A. disastrous.
B. unpredictable.
C. exciting.
D. colourful.