请阅读Passage 2，完成第小题。
Passage 2
That experiences influence subsequent behavior is evidence of an obvious but nevertheless remarkable activity called remembering. Learning could not occur without the function popularly named memory. Constant practice has such an effect on memory as to lead to skillful performance on the piano, to recitation of a poem, and even to reading and understanding these words. So-called intelligent behavior demands memory, remembering being a primary requirement for reasoning. The ability to solve any problem or even to recognize that a problem exists depends on memory.
Typically, the decision to cross a street is based on remembering many earlier experiences.
Practice (or review) tends to build and maintain memory for a task or for any learned material.
Over a period of no practice what has been learned tends to be forgotten; and the adaptive consequences may not seem obvious. Yet, dramatic instances of sudden forgetting can be seen to be adaptive. In this sense, the ability to forget can be interpreted to have survived through a process of natural selection in animals. Indeed, when one's memory of an emotionally painful experience leads to serious anxiety, forgetting may produce relief. Nevertheless, an evolutionary interpretation might make it difficult to understand how the commonly gradual process of forgetting survived natural selection.
In thinking about the evolution of memory together with all its possible aspects, it is helpful to consider what would happen if memories failed to fade. Forgetting clearly aids orientation in time,since old memories weaken and the new tend to stand out, providing clues for inferring duration.
Without forgetting, adaptive ability would suffer, for example, learned behavior that might have been correct a decade ago may no longer be. Cases are recorded of people who(by ordinary standards) forgot so little that their everyday activities were full of confusion. This forgetting seems to serve the survival of the individual and the species.
Another line of thought assumes a memory storage system of limited capacity that provides adaptive flexibility specifically through forgetting. In this view, continual adjustments are made between learning or memory storage (input) and forgetting (output). Indeed, there is evidence that the rate at which individuals forget is directly related to how much they have learned. Such data offers gross support of contemporary models of memory that assume an input-output balance.

The main purpose of this passage is to__________.
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A．interpret the function of forgetting
B．illustrate the process of adapting
C．explain the performance of memory
D．emphasize the importance of learning