In relation to the law of contract,explain the rules relating to：
(a)acceptance of an offer;
(b)revocation of an offer.
This question requires an explanation of the rules relating to the acceptance and revocation of offers in contract law.
(a)Acceptance is necessary for the formation of a contract. Once the offeree has accepted the terms offered, a contract comes into effect. Both parties are bound： the offeror can no longer withdraw their offer, nor can the offeree withdraw their acceptance. The rules relating to acceptance are：
(i)Acceptance must correspond with the terms of the offer. Thus, the offeree must not seek to introduce new contractual terms into their acceptance (Neale v Merrett (1930)).
(ii)A counter-offer does not constitute acceptance (Hyde v Wrench (1840)). Analogously, a conditional acceptance cannot create a contractual relationship (Winn v Bull (1877)).
(iii)Acceptance may be in the form of express words, either oral or written. Alternatively, acceptance may be implied from conduct (Brogden v Metropolitan Railway Co (1877)).
(iv)Generally, acceptance must be communicated to the offeror. Consequently, silence cannot amount to acceptance (Felthouse v Bindley (1863)).
(v)Communication of acceptance is not necessary, however, where the offeror has waived the right to receive communication. Thus in unilateral contracts, such as Carlill v Carbolic Smoke Ball Co (1893), acceptance occurred when the offeree performed the required act. Thus, in the Carlill case, Mrs Carlill did not have to inform the Smoke Ball Co that she had used their treatment.
(vi)Where acceptance is communicated through the postal service, then it is complete as soon as the letter, properly addressed and stamped, is posted. The contract is concluded even if the letter subsequently fails to reach the offeror(Adams v Lindsell (1818)). However, the postal rule will only apply where it is in the contemplation of the parties that the post will be used as the means of acceptance. If the parties have negotiated either face to face, in a shop, for example, or over the telephone, then it might not be reasonable for the offeree to use the post as a means of communicating their acceptance and they would not gain the benefit of the postal rule.
The postal rule applies equally to telegrams (Byrne v Van Tienhoven (1880)). It does not apply, however, when means of instantaneous communication are used (Entores v Miles Far East Corp (1955)).
In order to expressly exclude the operation of the postal rule, the offeror can insist that acceptance is only to be effective on receipt (Holwell Securities v Hughes(1974)). The offeror can also require that acceptance be communicated in a particular manner. Where the offeror does not insist that acceptance can only be made in the stated manner, then acceptance is effective if it is communicated in a way no less advantageous to the offeror (Yates Building Co v J Pulleyn& Sons (1975)).
(b)Revocation is the technical term for the cancellation of an offer and occurs when the offeror withdraws their offer. The rules relating to revocation are：
(i)An offer may be revoked at any time before acceptance. However, once revocation has occurred, it is no longer open to the offeree to accept the original offer (Routledge v Grant (1828)).
(ii)Revocation is not effective until it is actually received by the offeree. This means that the offeror must make sure that the offeree is made aware of the withdrawal of the offer, otherwise it might still be open to the offeree to accept the offer(Byrne v Tienhoven (1880)).
(iii)Communication of revocation may be made through a reliable third party. Where the offeree finds out about the withdrawal of the offer from a reliable third party, the revocation is effective and the offeree can no longer seek to accept the original offer (Dickinson v Dodds (1876)).
(iv)A promise to keep an offer open is only binding where there is a separate contract to that effect. Such an agreement is known as an option contract, and it must be supported by separate consideration for the promise to keep the offer open.
(v)In relation to unilateral contracts, i.e. a contract where one party promises something in return for some action on the part of another party, revocation is not permissible once the offeree has started performing the task requested (Errington v Errington & Woods (1952)).