Jimmy Carter has died at 100. His grandson said exercise was the key to the former president's longevity.

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• Former President Jimmy Carter's grandson credited his longevity to exercise.
• Carter died at 100 on Sunday and was the longest-living president in US history.
• But Jason Carter told Time before his grandfather died that he would have attributed his longevity to his marriage.

The grandson of late President Jimmy Carter said that his grandfather's exercise...