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2. It's three weeks since An last received a letter from her friend
   An hasn't.....................................................................................
3.When did you start practicing Karate?
  How long......................................
4.Mai and Nam last visited their grandparents in February
  Mai and Nam haven't.......................................................
5. When I last met Nga, she was 12 years old
  I haven't...................................................
6. He last had a cold last winter
  He hasn't...............................................