Mike is walking through a parking lot and finds Kathy lying unconscious. He puts her in his car and takes her to the hospital. The hospital saves her life and when she becomes conscious they present her with a bill.

Kathy has to pay based on a unilateral contract.

Kathy has to pay based on an implied in fact contract.

Kathy has to pay based on a quasi contract.

Mike has to pay because he brought her to the hospital.