Which of These Is an Example of a Tying Agreement

Which of these is an example of a tying agreement?

A) as part of purchasing a new computer, a buyer must also purchase a new monitor
B) when purchasing a new digital music player, a buyer is given the option of buying songs
C) when purchasing a used car, a buyer is offered a discount on new tires
D) when purchasing a couch, the buyer is given free throw pillows