Which of the Following Is an Example of Linkage

Multiple Choice

Which of the following is an example of linkage?

A) You and your neighbor agree to take turns shoveling your driveways on snow days.
B) You agree to drive your little brother to school,and your older brother agrees to do the dishes when it is your turn.
C) Your friend doesn't share his lunch with you so you refuse to share yours the next day.
D) The president of the United States enacts tariffs on steel so the Chinese premier also enacts tariffs on steel.