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OXFORD, UNITED KINGDOM - MAY 14: Britain's Prime Minister David Cameron speaks at a 'Stronger In' campaign rally at a school in Witney, on May 14, 2016 in Oxford, England. (Photo by Eddie Keogh - WPA Pool /Getty Images)

OXFORD, UNITED KINGDOM - MAY 14:  Britain's Prime Minister David Cameron speaks at a 'Stronger In' campaign rally at a school in Witney, on May 14, 2016 in Oxford, England. (Photo by Eddie Keogh - WPA Pool /Getty Images)