The author's main purpose in writing this passage is to ______.

A．forgetfulness is a response to learning
B．the memory storage system is an exactly balanced input--output system
C．memory is a compensation for forgetting
D．the Capacity of a memory storage system is limited because forgetting occurs

A．forgetting for lack of practice tends to be obviously inadaptive
B．if a person gets very forgetful all of a sudden he must be very adaptive
C．the gradual process of forgetting is an indication of an individual's adaptability
D．sudden forgetting may bring about adaptive consequences