Our memories are fantastically complex, but anyone who has ever used a personal computer h

as a ready-made model with which to compare them. The computer model is not perfect, however. For a start, the largest mainframe. (主计算机)in the world can not compete with the potential brain power of human beings. Packed into even the thickest of human skulls are some 1,000 billion cells, or neurons, and each one can connect with thousands of its neighbors. Each connection represents a 'bit' of information and, in theory, we can carry more bits than there are atoms in the known universe.

There are two quite distinct types of memory-short-term and long-term. Our short-term memories are those which we hold on to for just as long as we need them. The vast majority of our everyday thoughts, sights and impressions are registered in the short-term memory only. They take the form. of patterns, or linked pathways, created by circulating currents of electrical energy. So long as the current is buzzing around its little route, the memory that it represents stays in the mind. But once the current dies down, the memory, too, starts to fade. While short-term memories consist of active electrical circuits, long-term memories are quite literally etched (铭刻) into our brains. It seems that if the pathways taken by a particular electrical current are well-trodden, or if the current passing along them is strong enough, the cells along the way change, so that the route or pattern is permanently marked.

Events which have strong meaning for us are particularly likely to be upgraded into the long-term memory. Part of the reason is probably that we go over and over these memories, keeping the pathways stimulated and the electrical current high. It's also likely that certain chemicals come into play, too. When we are excited, very happy, or frightened, our glands pump out chemicals such as adrenaline. One theory has it that some of these chemicals stimulate the neurons to alter their structure and forge permanent connections with each other.

It follows, then, that a memory formed when we are 'up' is more likely to stick than one registered when we are down. Several experiments seem to bear this out. In one, a group of students was first shown an exciting film, then given a list of words to memorize. Another group was shown a miserable film, then given the same learning task. Next day the group who watched the happy film could remember 20% more words than the other group.

About the memory in comparison with the computer the writer says that ______.

A．neither of them is completely reliable all the time

B．the memory is more complicated than the computer

C．the computer operates in a more organized way than the memory

D．neither of them is used to its fullest capacity all the time