The method of zone refining of metals is based on the principle of:

$MgO$ is used as a refractory material because

Consider the following statements :

$(a)$ When the lead-silver alloy rich in silver, lead is removed by the cupellation process

$(b)$ Froth flotation can be applied for non sulphide ore also using suitable activator

$(c)$ $PbSO_4 + H_2SO_4$, electrolyte is used for the refining of $Pb$ by electrolysis

$(d)$ Any metal will not reduce the oxide of other metals which lie above it in the Ellingham diagram.

Using $'T'$ for true and $'F'$ for false statements in the given sequence, select the correct set of code

The metal $X$ is prepared by the electrolysis of fused chloride. It reacts with hydrogen to form a colourless solid from which hydrogen gas is released on treatment with water. The metal is

The metal extracted by leaching with cyanide is