$2 \, mL$ each of concentrated $HCl$,$HNO_3$,and a mixture of concentrated $HCl$ and concentrated $HNO_3$ in the ratio of $3:1$ were taken in test tubes labeled as $A$,$B$,and $C$. $A$ small piece of metal was put in each test tube. No change occurred in test tubes $A$ and $B$,but the metal dissolved in test tube $C$. The metal could be:

$(a)$ $A$ substance $X$,an oxide of a metal,is used extensively in the cement industry. This element is found in our bones also. On treatment with water,it forms a solution which turns red litmus blue. Identify $X$ and also write the chemical reaction involved.
$(b)$ Choose a metal from the following metals which reacts only with hot water: Sodium,magnesium,iron. Mention the products formed during the reaction.

Which alloy is used to solder electric wires?

Which metal is purified by the method of liquation?

What is the basic raw material used in the Contact Process for the production of sulfuric acid?

What type of oxide is aluminium oxide $(Al_2O_3)$?