$6.$ Joe sells widgets. He has been trying to sell his products to Tom for some time. Joe has made his final sales pitch to Tom, offering $10$ per cent off of the list price, $60$ days for payment, a delivery date and delivery terms. What has happened to this point in the negotiation process?$($a$)$ The parties have agreed on a contract.$($b$)$ The parties have agreed on an offer$($c$)$ A complete offer has been made by Joe that can be accepted by Tom$($d$)$ The parties have agreed to do business