According to data from the U.S. Department of Energy, sales of the fuel-efficient Toyota Prius hybrid fell from 158,574 vehicles sold in 2008 to 139,682 in 2009. Over the same period, according to data from the U.S. Energy Information Administration, the average price of regular gasoline fell from $3.27 to $2.35 per gallon. Using the midpoint method, calculate the cross-price elasticity of demand between Toyota Prii (the official plural of "Prius" is "Prii") and regular gasoline. According to your estimate of the cross-price elasticity, are the two goods complements or substitutes? Does your answer make sense?