若方程x2-5x+6=0和方程x2-x-2=0的所有解构成的集合为M，则M中元素

下列叙述正确的是  （    ）

A．自然界的物质都以化合物的形式存在

B．凡是含氧元素的化合物都是氧化物

C．冰块和水混合得到混合物

D．同种元素组成的纯净物是单质

情境教学法所属的教学方法类型是( )

A．以实际训练为主的

B．以直观感知为主的

C．以情感陶冶为主的

D．以探究活动为主的

Caz is a director of Dull plc, but she also carries out her own business as a wholesale supplier of specialist metals under the name of Era Ltd.Last year Dull plc entered into a contract to buy a large consignment of metal from Era Ltd. Caz attended the board meeting that approved the contract and voted in favour of it, without revealing any link with Era Ltd.Required:Analyse the situation explaining any potential liability that Caz may have in relation to the sale of the metal to Dull plc by Era Ltd.(10 marks)

热水管道应在管道最高点设（）。

A.安全阀

B.排水阀

C.放气阀

D.吹扫阀

信号电缆使用前应进行哪些测试？