The average cost of a gallon of regular gasoline in the U.S. rose $0.05, to $3.56, during the past two weeks. That's according to a survey released Sunday.

Industry analyst Trilby Lundberg says the increase stems from a jump in the price of ethanol used to make gasoline.

She says another reason for the rise is that U.S. refiners are starting to switch over to more expensive summer-blend gasoline.

The average prices of a gallon of midgrade and premium gas were $3.74 and $3.89, respectively. Diesel averaged $4.02 per gallon.

Of the cities surveyed in the Lower 48 states, Billings, Mont., has the lowest average price at $3.18 a gallon and Los Angeles has the highest average price at $4.

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