Some other options for answers:

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#2. People visit Washington D.C., in the spring.  They can see the cherry trees in bloom then.

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People visit Washington, D.C. in the spring, when they can see the cherry trees in bloom. 

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People who visit Washington, D.C., in the spring can see the cherry trees in bloom.  

(I sometimes get this answer from students.  It is o.k., but the sentence has a defining relative clause ("who visit Washington, D.C.") and the instructions say to use non-defining here.  

 

#3. The cherry trees in Washington, D.C., were a gift from the Japanese government to the U.S. They are admired by everyone.

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The cherry trees in Washington, D.C., which were a gift from the Japanese government to the U.S., are admired by everyone.

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The cherry trees in Washington, D.C., which are admired by everyone, were a gift from the Japanese government to the U.S.

 

p. 14  Vocabulary:

1. transportation system

2. cuisines

3. cost of living

4. nightlife

5. climate

6. landmarks

7. green spaces