11.A new study on birds’ sleep has revealed that .

　　A) half-brain sleep is found in a wide variety of birds

　　B) half-brain sleep is characterized by slow brain waves

　　C) birds can control their half-brain sleep consciously

　　D) birds seldom sleep with the whole of their brain at rest

　　12.According to the passage, birds often half sleep because .

　　A) they have to watch out for possible attacks

　　B) their brain hemispheres take turns to rest

　　C) the two halves of their brain are differently structured

　　D) they have to constantly keep an eye on their companions

　　13.The example of a bird sleeping in front of a mirror indicates that .

　　A) the phenomenon of birds dozing in pairs is widespread

　　B) birds prefer to sleep in pairs for the sake of security

　　C) even an imagined companion gives the bird a sense of security

　　D) a single pet bird enjoys seeing its own reflection in the mirror

　　14.While sleeping, some water mammals tend to keep half awake in order to .

　　A) alert themselves to the approaching enemy

　　B) emerge from water now and then to breathe

　　C) be sensitive to the ever-changing environment

　　D) avoid being swept away by rapid currents

　　15.By “just the tip of the iceberg” (Line 2, Para. 8), Siegel suggests that .

　　A) half-brain sleep has something to do with icy weather

　　B) the mystery of half-brain sleep is close to being sleepers

　　C) most birds living in cold regions tend to be half sleepers

　　D) half-brain sleep is a phenomenon that could exist among other species