参考答案：

 Part Two of the United Nations Convention for the International Sale of Goods provides that the formation of the contract is concluded through the exchange of an offer, followed by due acceptance of that offer. Once an offer has been accepted, a contract comes into existence and the parties are bound by the terms of their agreement. Article AD(A) of the convention provides that:‘A proposal for concluding a contract addressed to one or more specific persons constitutes an offer if it is sufficiently definite and indicates the intention of the offeror to be bound in case of acceptance. A proposal is sufficiently definite if it indicates the goods and expressly or implicitly fixes or makes provision for determining the quantity and the price.’However, for a proposal for concluding a contract to constitute an offer it must be sufficiently definite. This requires that the offer must indicate the goods to be transferred and either expressly or implicitly fix or make provision for determining the quantity of the goods to be transferred and the price to be paid. Any communication that does not comply with the stated requirements for an offer is consequently to be treated as merely an invitation to make offers. As a consequence, it is for the recipient of the invitation to make the acutal offer to the first party who then is in the position to either accept or reject the proposal from them. Particular problems arise where a purported acceptance of an offer contains additional or different terms than those proposed in the original offer. Article AI provides that: (A) A reply to an offer which purports to be an acceptance but contains additions, limitations or other modifications is a rejection of the offer and constitutes a counter-offer. (B) However, a reply to an offer which purports to be an acceptance but contains additional or different terms which do not materially alter the terms of the offer constitutes an acceptance, unless the offeror, without undue delay, objects orally to the discrepancy or despatches a notice to that effect. If he does not so object, the terms of the contract are the terms of the offer with the modifications contained in the acceptance. (C) Additional or different terms relating, among other things, to the price, payment, quality and quantity of the goods, place and time of delivery, extent of one party’s liability to the other or the settlement of disputes are considered to alter the terms of the offer materially. Consequently under the Convention, if the additional or different terms do not materially alter the terms of the offer, the reply will constitute an acceptance. However, even in relation to such non-material alterations, the offeror can object to the new terms as long as they respond without undue delay. If they do not object, the terms of the contract are the terms of the offer with the modifications contained in the acceptance. If the additional or different terms do materially alter the terms of the contract, the reply constitutes a counter-offer that must in turn be accepted by the original offeror for a contract to come into effect. Applying the convention rules to the facts of the scenario it can be seen that at no time did a contract come into being. Ax’s first contact with Bo merely solicited information about the possibility of their providing the screws and Bo’s response supplied that information, but did not amount to an offer to supply the screws. Ax’s next communication did amount to an offer to purchase both the screws and the nuts and bolts, but it was not accepted by Bo, rather he introduced a new term into the agreement, the need for prepayment, or payment through a letter of credit. The subsequent invoice also introduced a new term into the negotiations relating to the lower quality of the screws to be supplied. This document constituted a new offer from Bo, which Ax could either accept or reject. When Ax objected to the new terms he was effectively making a new offer, which it was then up to Bo to accept or reject. By insisting on the higher price for the higher quality he effectively rejected Ax’s offer and has restated his own offer.