When the 20 students from Mrs. McCabes class returned from summer vacatin, they decided to celebrate by working off some energy. They came up with a plan: The first student ran down the row of lockers and opened every door. The second student started with locker #2 and closed every second door. The third student started with locker #3 and changed the state of every third locker door. The fourth student started with locker #4 and changes the state of every fourth locker, the fifth student started with locker #5 and chanded the of every fifth locker, so on, until all 20 students passed. Which lockers are still open after the twentieth student is finished?

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