19) D

 Julie put her clock 1 hour forward. Since she was meant to put her clock 1 hour back, difference between the actual time and her time = 1 – ( – 1) = 1 + 1 = 2 hours. So when her clock showed 11 am, the actual time must have been = 11 – 2 = 9 am.

Clare did not interfere with her clock. So when the time is 11 am, her clock would have shown 10 am. So she would have arrived at 10 am.

So Julie arrives 1 hour before Clare.

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