$6.$ A contract does not come into existence until an offer has been made by one party and accepted by the other party. From the choices below, please select one that constitutes an offer whereby the offeror is bound when the offer is accepted.

A$.$ The display of a coat in a store window.

B$.$ An invitation to a friend to do business.

C$.$ An advertisement to sell goods at a certain price.

D$.$ An advertisement to sell a fixed number of items at a fixes price to those who accept first. An offer of a reward for information or for return of a lost object.