Brittney offers to pay Sal $500 if Sal will paint her house while she is out of town on vacation for two weeks. Sal makes no promise but tells Brittney that she will think about it. While Brittney is out of town, Sal paints the house. This is best described as a[n]:

Select one:

a.

quasi contract.

b.

unilateral contract.

c.

implied in fact contract.

d.

bilateral contract.