There are as many different types of sleeper as there are animal varieties on the planet.There are dozers，dreamers，nappers and hardcore dead-to-the-worlders.
   Dolphins
   Dolphins face a bit of a problem as they spend their whole lives in the water，but as mammals they need to breathe oxygen to survive.How do they juggle this when unconscious？Dolphins shut down half of their brain，which rests，while the other half is alert and looking out for any potential dangers.As they do this，the dolphin will lie on the surface of the water in a behaviour known as "logging".
   Elephants
   Elephants tend to nod off for a few minutes at a time throughout the day.Elephants only manage about two hours of sleep per day，and that's not even a concentrated block of shut-eye.They tend to nod off for a few minutes at a time throughout the day.Possibly because of this，most of their sleeping is done standing up，with just the occasional recumbent （伏臥的）sleep.
   Spiders
   Of course spiders have to be awkward.Even though animals sleep in a vast array of different ways，all of them，even bacteria，follow a 24 hour cycle，known as a circadian rhythm that is dictated by the rotation of the Earth.But scientists have discovered that certain types of orb-weaving spiders simply don't bother with this.Their body clocks follow a 17，18 or 19 hour cycle.Scientists aren't exactly sure how the spider can even exist with this completely different body clock.
    Koalas
   One problem with trying to work out a creature's sleeping habits is the difficulty of identifying what is actual sleep.It's easier to study captive animals，but they can behave in a completely different manner to their cousins in the wild.Koalas studied in zoos were thought to sleep for up to 22 hours a day，but a more detailed study，looking at animals in the wild too，revealed that they in fact sleep for around 14 hours a day plus a healthy amount of resting on top of that.Their diet of eucalyptus leaves takes a lot of time and energy to digest.
   Frogs
   Frogs are incredible.They have managed to master practically every environment on the planet，no matter how hot or how cold.They not only use hibernation in winter as a sleep strategy，they also engage in something called estivation，where they reach a dormant state during the summer months to avoid overly hot or dry conditions.Despite all this knowledge，scientists aren't sure exactly how frogs sleep during the months when they are active.

（1）Which animal sleeps the longest hours a day according to the passage？

B

B

A．The dolphin.
B．The koala.
C．The spider.
D．The elephant.
（2）What is still confusing to scientists？

D

D

A．How they can identify what is actual sleep.
B．How dolphins breathe oxygen when sleeping.
C．How spiders can survive a different body clock.
D．How frogs sleep when they are not hibernating or estivating.
（3）What is the passage mainly about？

C

C

A．Why animals sleep differently.
B．What animals do while sleeping.
C．The extraordinary ways that animals sleep.
D．The mystery about different types of sleeper.