Answer:
In the article since Toyota Prius is partly run by fuel it's sale is hurt by lowering of gasoline price. So more gasoline leads to less Toyota Prius sales. This indicates substitution.
On the other hand considering wholly gasoline powered cars, when prices of gasoline goes down, more gasoline is purchased and people tend to trade in their Toyota Prius for gasoline cars. In this case relationship is complimentary as increase in sale of gasoline leads to increase in sale of gasoline cars.
Explanation:
