1 The river is dirty.it has been dirty since two years ago.
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The river is dirty.it has been dirty for two years .
2 The book is interesting.I have kept it since a week ago.
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The book is interesting.I have kept it for a week .
3 He is not in beijing now.he has been away from beijing since two hours ago.
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He is not in beijing now.he has been away from beijing for two hours .
4 Tina and martin live happily,they have been married since 2008.
5 Tom likes playing soccer.he has been in the soccer club since Monday.