Abel had grown tired of working on the family farm and was considering moving away. However, his parents told him "If you'll stay and help us, we'll leave you the farm when we die." As a result, Abel decided to stay on the farm and worked there for the next twenty years. However, when his parents finally died Abel learned that they had left the farm to his brother, Mark. Abel filed suit against his parents' estate to enforce their promise of leaving him the farm. Which of the following is a true statement?
A) Abel gave no consideration for his parents' promise to leave him the farm, so their promise is not enforceable against the estate.
B) The work that Abel did on the farm before his parents made their promise is sufficient consideration to make their promise enforceable.
C) Abel's parents' promise is not enforceable because it was illusory.
D) Abel's parents' promise is enforceable under a promissory estoppel theory.