下列4个命题，其中错误的命题是（　　）

A．对角线相等的梯形是等腰梯形

B．对角线互相垂直且相等的四边形是正方形

C．斜边和一个锐角对应相等的两个直角三角形全等

D．等腰三角形底边上的中点到两腰的距离相等

(c) Explain the application of the ‘two thirds rule’ in relation to a composite supply of goods and services, indicating how a trader may seek to minimise its effects. (5 marks)