There are 13 trees on one side of the street on Trina's way from her house to school. Today, on her way to school, Trina put a chalk mark on every other tree, starting with the first she passed. When she goes home from school, she will put a chalk mark on every third tree, again starting with the first one she passes. By the time Trina arrives at home, how many of the 13 trees will not have a chalk mark on them?
(A) 6 (B) 4 (C) 8 (D) 2 (E) 7
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Source: CEMC, University of Waterloo,
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