Answer:
The answer is the foot-in-the-door phenomenon.
Explanation:
This persuasion technique consists in asking for a small request, followed by a larger one that will likely be accepted. This can be explained because the person requested feels socially responsible for complying after a smaller favour has been already accepted.
Naturally, this technique is extensively used in marketing. In fact, the name itself refers to a salesperson placing his foot on the door to prevent it from closing.
