A、An introduction to two kinds of tools. B、The origin of the silicon chips. C、The way ancient people made tools. D、The changes b

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. 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. 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 fingernail, 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. Human beings used stone chips for more than two million years, but human life changed very little at that time. We have used silicon chips for only a few years, but life is changing faster every day.
23.How did early human beings make tools two million years ago?
24.What’s the difference between the stone chips and the silicon chips?
25.What is the passage mainly about?