You post a flyer in your neighborhood to paint the house of the first person that brings $500 to your front door. The next day, somebody has left an envelope taped to your front door containing $500 in cash, along with a copy of your flyer with their home address and the words "barn red" written on the back.

Choose the correct contract type and explain why

a. Express Bilateral Contract

b. Express Unilateral Contract

c. Implied Bilateral Contract

d. Implied Unilateral Contract

e. No Contract/Unenforceable Contract