A national survey finds the average U.S. price of regular-grade gasoline has plummeted 9 cents a gallon over the past two weeks to $3.58.  That’s the largest drop this year.  Industry analyst Trilby Lundberg said Sunday the decrease came despite a rise in crude-oil prices.  Lundberg says U.S. refiners, enjoying plentiful supplies, aggressively cut wholesale prices to chase sales.  The U.S. average retail diesel price is down 4 cents per gallon, to $3.90.  The lowest average price Lundberg found in the lower 48 states was $3.23 in Tulsa, Oklahoma. The highest was $4.03 in San Francisco.  In Illinois, the average price per gallon of regular gas is at $3.59.  Diesel averaged at $3.38 a gallon.