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A water retention pond stands near oil storage tanks at the Enbridge Inc. Cushing storage terminal in Cushing, Oklahoma, U.S., on Wednesday, March 25, 2015. The fastest oil-inventory growth on record at the main U.S. hub may be about to end, easing concern that storage limits will be strained. Photographer: Daniel Acker/Bloomberg via Getty Images

A water retention pond stands near oil storage tanks at the Enbridge Inc. Cushing storage terminal in Cushing, Oklahoma, U.S., on Wednesday, March 25, 2015. The fastest oil-inventory growth on record at the main U.S. hub may be about to end, easing concern that storage limits will be strained. Photographer: Daniel Acker/Bloomberg via Getty Images