Bone black is a form of which element?

Which of the following units is used to form cross-linked silicone?

An example of a neutral oxide is

The basic building unit of all silicates is

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 $Z$?

Describe the preparation and properties of $CO_2$.