A、It was one of the first tools. B、It developed human intelligence. C、It led to the invention of machines. D、It was the key to t

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 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 success of mankind.
    Since 1960 a new kind of tool has appeared. This is the silicon chip—a little chip of silicon crystal. It is smaller than a finger-nail, but it can store more than a million "bits" of information. It is an electronic brain.
    Every year these chips get cleverer but their size gets smaller, and their cost gets less. They are used in watches, calculators and intelligent machines that we can use in many ways.
    In the future we will not need to work with tools in the old way. Machines will do everything for us. They will even talk and play games with us. People will have plenty of spare time. But what will they do with it?