Answer:
MD did not revoke their offer, so the contract was valid.
Explanation:
First of all, in order for an offer to be revoked, it must be done before the other party accepts the offer. Once the other party accepted the offer, a contract is formed. E.g. the offeror made a promise but revoked it before the offeree accepted, it is not valid anymore. But this didn't happen in this case, both sides agreed on the offer, so a binding contract exists.
MLS cannot revoke its offer at will simply because they want to, specially after receiving something of value in exchange. MD had already began working, so they had already given consideration to MLS.
In order for a revocation to be valid, it must be communicated to the other party before the acceptance, and that didn't happen here. Just because someone in MLS shouted out loud or told a friend that the offer was revoked, doesn't revoke it.