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．
Questions 16-20 are based on Passage Four.
16. From the evolutionary point of view,_______.
A．sudden forgetting may bring about adaptive consequences
B．forgetting for lack of practice tends to be obviously inadaptive
C．if a person gets very forgetful all of a sudden he must be very adaptive
D．forgetting is an indication of an individual’s adaptability
17.According to the passage, if a person never forgot,_______.
A．he would survive best
B．he would have a lot of trouble
C．the evolution of memory would stop
D．his ability to learn would be enhanced
18.From the last paragraph we know that _______.
A．forgetfulness is a response to learning
B．memory is a compensation for forgetting
C．the capacity of a memory storage system is limited
D．the memory storage system is balanced
19.The tone of the passage can best be described as _______.
A．humorous B．theoretical
C．exaggerative D．philosophical
20.The author’s main purpose in writing this passage is to _______.
A．interpret the function of forgetting
B．illustrate the process of adapting
C．explain the performance of memory
D．emphasize the importance of learning
Passage Five
Many people believe that beavers are intelligent animals. After all, their dams are fine examples of engineering. The engineering feats of beavers are well known, but it will be useful to recall their main features.
A pair of beavers will construct a dam across a river. The water held back by the dam overflows the bank on either side of the river, flooding the adjacent ground and forming a pond. At some point in the pond the beavers then build their home, which is called a lodge. This consists of a conical pile of branches and sticks of two to six feet in length held together with mud and stones, the top of which projects above the waterline. It serves as a shelter from the elements, a refuge from enemies and a base for food supplies to be drawn upon in winter.
From an engineering point of view the lodge could hardly be improved．Not only does it contain a central chamber just above water level，but it also has one or more escape tunnels，well-insulated walls and a vertical chimney，which regulates the temperature inside and gives air-conditioning．It is altogether a cunning piece of construction，with all modern conveniences．It is，in fact，better protected against the effects of flooding than many human habitations．
