50（）

A.them
B.they
C.theirs
D.their

Long long ago mankind began to use tools. Throughout the world, you can still find the tools which were used over two million years ago. People made such tools by hitting one stone (36) another. By this means, they got pieces from one of the stones. These pieces of stone were always (37) on one side. People utilized them to (38) skin and meat from some animals that were dead, (39) to make other tools from wood. Mankind needed (40) use tools since, unlike other meat-eating (41) , say lions, they did not have sharp teeth. Tools greatly helped people obtain food more (42) .

Tools helped (43) human intelce as well when human worked with them. The human brain became bigger gradually, and human beings began to invent a growing number of tools. The stone piece was one of the (44) tools that were used by people, and maybe it was (45) . Some scientists hold it was the key (46) human beings’ success.

A new kind of tool has come into people’s view since 1960, that is, the silicon chip a small chip of silicon crystal. It is a bit (47) than a fingernail, but it is able to (48) several million "bits" of information or so. No doubt that it is an (49) brain.

Each year these chips grow cleverer, however (50) size becomes smaller, what’s more, their cost gets (51) .

It is (52) over two million years that human beings used chips, but human (53) changed so little at that time. Now we have made use of silicon chips for merely (54) years, but our life is changing faster with each passing day.

(55) will human life be like in two decades