Which of the following is an example of the preexisting duty rule?

Aubrey entered into an agreement with Ping in which Aubrey would deliver the computers that Ping made in consideration of $$25$ per delivery

Kristi was sitting at a bus stop when she saw Tim absent$-$mindedly walking into traffic. She warned Tim, and he stepped back just in time. Tim offered Kristi $$1,000$ in consideration.

All of the hypotheticals are examples of the preexisting duty rule

Deputy Kelly entered into a written agreement with Babe in which Kelly agreed to find the person who stole Babe's skateboard in consideration of $$100$