Which of the Following Is an Example of Reciprocity

Which of the following is an example of reciprocity?

A) Kennedy reveals a competitor's secret formula for a plastic hardening compound.
B) Larry's Uniform Company agrees to buy Harry's landscaping services if Harry will rent its uniforms from Larry.
C) When the purchasing agent won't see Otto, he bribes the receptionist to get him an appointment with the company's CEO.
D) In order to carry a line of profitable wind chimes, Scott's Hardware Store also has to carry a less profitable line of hose nozzles made by the same company.
E) A group of kaolin (used in the whitening of paper) manufacturers agree to a price freeze.