In economics, goods are substitutes if a DIRECT relationship exists between the change in Quantity Demanded of one good and the change in price of a second good.
If Price of Good 1 increases and the Quantity Demanded for Good 2 increases, and vice versa, if the Price of Good 2 decreases and the Quantity Demanded for Good 2 decreases then the goods are Substitutes.
You can see as the price of beef increases the demand for Chicken and Pork increases as well. Now, there could be other reasons the demand for chicken/pork has increased other than the price of beef.
There is your homework. Get to work! :)
|Source: Wall Street Journal|