A news story titled "Auto Sales Zoom, Helped by Low Prices at the Pump" includes a graphic showing increasing car and truck sales in one panel and decreasing gasoline prices in the other panel. Source: "Auto Sales Zoom, Helped by Low Prices at the Pump," Wall Street Journal, December 3, 2014. The graphic described assumes that autos and gasoline are
A. complements, because if the price of gasoline decreases, the demand for its complements, cars and trucks, increase.
B. complements, because if the price of gasoline increases, the demand for its complements, cars and trucks, increase.
C. substitutes, because if the price of gasoline decreases, the demand for its substitutes, cars and trucks, decrease.
D. substitutes, because if the price of gasoline decreases, the demand for its substitutes, cars and trucks, increase.

A decrease in the price of gasoline will increase the demand for automobiles. This is both are complements. Complementary goods have a negative cross-elasticity of demand. So a fall in the price of a good will cause an increase in the demand for its complements.

The graphic described here indicates that automobiles and gasoline are complements.