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'Do we want his finger anywhere near the button?' Hillary Clinton asked during the US election campaign, referring to Mr Trump and the nuclear arsenal. But how close is an American president's finger to 'the button'? How close is anyone’s? We explain how the nuclear weapons systems of the US, Russia, Britain and China work – and how much power any one individual has over them. (Photo: A Trident Ii, Or D-5 Missile, Is Launched From An Ohio-Class Submarine. Credit: Getty Images)