In atomic reactors,graphite is used as a

Which of the following halides is the most stable?

The butter of tin is represented by

Among the following,the correct statements are:
$I$. Germanium exists only in traces.
$II$. $PbF_4$ molecule is tetrahedral in shape.
$III$. $GeX_2$ is more stable than $GeX_4$.

In the laboratory,silicon can be prepared by the reaction:

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