乳牙的大面积龋坏，不但会使牙弓长度减少，还会影响牙合间高度，为确保乳牙牙合的正常宽度的高度，对于多个面龋坏的牙齿应用（）

A.不锈钢合金冠修复牙体

B.远中导极保持器

C.功能性活动保持器

D.局部固定式间隙扩展装置

E.舌弓保持器

Alvin runs a business selling expensive cars. Last Monday he mistakenly placed a notice on one car indicating that it was for sale for ￡5,000 when in fact its real price was ￡25,000. Bert later noticed the sign and, recognising what a bargain it was, immediately indicated to Alvin that he accepted the offer and would take the car for the indicated amount. Alvin, however, told Bert that there had been a mistake and that the true price of the car was ￡25,000. Bert insisted that he was entitled to get the car at the lower price, and when Alvin would not give it to him at that price Bert said that he would sue Alvin.After Bert had left, Alvin changed the price on the car to ￡25,000 and subsequently Cat came in and said she would like to buy the car, but that she would have to arrange finance.On Tuesday Del came by and offered Alvin the full ￡25,000 cash there and then and Alvin sold it to him.Required:Advise Alvin, Bert, Cat and Del as to their rights and liabilities in the law of contract. (10 marks)