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51.__________,

The Polar Bears like to swim all the year round，for_____．

• A、they can remain young
  

• B、it is an easy way to keep the body warm in winter
  

• C、they find it enjoyable and interesting
  

• D、they might meet fewer troubles in life

Scientists have discovered that what prevents people from concentrating is_____.

• A、any kind of noise
  

• B、great changes in level of  noise
  

• C、background noise
  

• D、various background music