How is $H_2S$ liberated in the laboratory?

Explain the following reactions:
$(a)$ Silicon is heated with methyl chloride at high temperature in the presence of copper.
$(b)$ Silicon dioxide is treated with hydrogen fluoride.
$(c)$ $CO$ is heated with $ZnO$.
$(d)$ Hydrated alumina is treated with aqueous $NaOH$ solution.

Identify the species,which does not exist?

An allotrope of carbon which exhibits only two types of $C-C$ bond distances of $143.5 \text{ pm}$ and $138.3 \text{ pm}$, is $....$

$SiCl_4$ on hydrolysis forms '$X$' and $HCl$. Compound '$X$' loses water at $1000^{\circ} C$ and gives '$Y$'. Compounds '$X$' and '$Y$' respectively are:

The plague of tin is the: