Which one of the following is a semiconductor?

$A$ certain element forms a solid oxide which when dissolved in water forms an acidic solution. The element is:

$Si_2O_7^{6-}$ anion is obtained when

What is meant by $Tin \ Plague$?

Choose the correct statement from the following with reference to the inert pair effect.

The element $X$ on reaction with conc. $HNO_3$ forms two acidic oxides $A$ and $B$ of different shapes. Element $Z$ on reaction with conc. $H_2SO_4$ forms two acidic oxides $A$ and $D$ of different shapes. What are $X$ and $Z$?