Prudence,a Mean Business Law Teacher,offers $50 to Any Student Who

Prudence,a mean business law teacher,offers $50 to any student who will mow her lawn.Sam goes right over and begins mowing.Just before he finishes,Prudence goes over and tells him that she revokes her offer.Which of the following is true regarding Sam's entitlement to payment?

A) Sam is not entitled to payment because a bilateral contract was involved that could only be accepted by full performance prior to revocation.
B) Sam is not entitled to payment because a unilateral contract was involved that could only be accepted by full performance prior to revocation.
C) Sam is entitled to only a proportional recovery based on the amount of work he had done prior to the revocation because Prudence always retained the right to validly revoke.
D) Sam is entitled to recover because a unilateral contract was involved, and he was entitled to a reasonable amount of time to complete the job.
E) Sam is entitled to recover because a bilateral contract was involved, and he was entitled to a reasonable amount of time to complete the job.