When Sam's regular nurse told Sam that she required back surgery and that the doctor estimated it would take at least six months for her to recover sufficiently to resume work, Sam sought to hire Jill to be his nurse for a period of up to six months, or until Sam's regular nurse became physically able to resume her duties. In exchange for Jill's services, Sam promised to either (1) pay Jill $5,000 per month or (2) provide Jill's family with suitable rent-free housing for as long as Jill was Sam's nurse.

Question:

Sam made alternative promises to Jill as consideration for her services. Sam was able to choose between the two alternatives, yet a valid contract was formed. Why?