单选题 .  It is difficult to imagine what life would be like without memory. The meanings of thousands of everyday perceptions, the bases for the decisions we make and the roots of our habits and skills are to be found in our past experiences, which are brought into the present by memory.
    Memory can be defined as the capacity to keep information available for later use. It includes not only "remembering" things like arithmetic or historical facts, but also involves any change in the way an animal typically behaves. Memory is involved when a rat gives up eating grain because he has sniffed something suspicious in the grain pile. Memory is also involved when a six-year-old child learns to swing a baseball bat.
    Memory exists not only in humans and animals but also in some physical objects and machines. Computers, for example, contain devices for storing data for later use. It is interesting to compare the memory-storage capacity of a computer with that of a human being. The instant-access memory of a large computer may hold up to 100,000 "words"—ready for instant use. An average U.S. teenager probably recognizes the meaning of about 100,000 words of English. However, this is but a fraction of the total amount of information which the teenager has stored. Consider, for example, the number of faces and places that the teenager can recognize on sight.
    The use of words is the basis of the advanced problem-solving intelligence of human beings. A large part of a person's memory is in terms of words and combinations of words.1.  According to the passage, memory is considered to be ______.