将硝酸铵溶于冷水后立即灌入橡皮袋中，可起应急制冷作用．干冰也可作制冷剂．请简述两者的制冷原理．

In relation to the law of contract:

(b) Explain the ways in which an offer may be revoked as provided under the Contracts Act 1950. (5 marks)