HELP! Multiple Choice Business Law?

[May L]

wHEN Mary loses her diamond ring at the annual Garden Show, she prepares a notice of $500 reward. Before she can post or contact the newspaper, Bob finds the ring in the water bucket and return it to her. This is not a valid contract because
a. the offer wasn't communicated to the offeree
b. there was no acceptance
c. there is no legal detriment suffered by Bob
d.none above

Milly discover a china vase with pink rosebud patterns. Due to the abundance of junk she found, Milly decide to have a yard sale. Florence though the vase would look nice in her daughter room and offere to buy the vase for $5. Milly accept. Two months later, Milly saw a similar vase in an antique shop selling fro $750. The contract is
a. invalid because it lacks sufficiant consideration
b. valid and enforceable
c. executory
d. invalid on the basis of mistake