共用题干
When We Are Asleep

Everyone dreams,but some people never recall their dreams,or do so very rarely.
Other people always wake up with vivid recollections（记忆）of their dreams, though they
forget them very quickly.In an average night of eight hours' sleep,an average adult will
dream for around one hundred minutes,probably having three to five dreams,each lasting
from ten to thirty minutes.
Scientists can detect when someone is having a dream by using an instrument which
measures the electrical waves in the brain.During dreaming,these waves move more
quickly.Breathing and pulse rate also increase,and there are rapid eye movements under
the lids，just as though the dreamer were really looking at moving objects.These signs of
dreaming have been detected in all mammals（哺乳动物）studied, including dogs,
monkeys, cats, and elephants, and also some birds and reptiles（爬行动物）.This period
of sleep is called the"D"state.Babies experience the"D"state for around 50%of their
sleep;the period reduces to around 25%by the age of 10.
Dreams take the form of stories,but they may be strange and with incidents not
connected,which make little sense.Dreams are seldom without people in them and they
are usually about people we know.One estimate says that two-thirds of the"cast"of our
dream dramas are friends and relations.Vision seems an essential part of dreams,except
for people blind from birth.Sound and touch are senses also often aroused,but smell and
taste are not frequently involved.In"normal"dreams,the dreamer may be taking part,or
be only an observer. But he or she cannot control what happens in the dream.
However,the dreamer does have control over one type of dream.This type of dream is
called a "lucid"（清醒的）dream. Not everyone is a lucid dreamer. Some people are
occasional lucid dreamers.Others can dream lucidly more or less all the time.In a lucid
dream,the dreamer knows that he is dreaming.

In a lucid dream we can use Morse code to communicate with others.
A:Right
B:Wrong
C:Not mentioned