Which of the following is true for diamond?

Assertion : Stannous chloride gives grey precipitate with mercuric chloride,but stannic chloride does not do so.
Reason : Stannous chloride is a powerful oxidising agent which oxidises mercuric chloride to metallic mercury.

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 $GeX_4$.

$Ge(II)$ compounds are strong reducing agents,whereas $Pb(IV)$ compounds are strong oxidizing agents. This is due to:

Mention the uses of pure forms of germanium and silicon.

The silicates which contain $Si_2O_7^{6-}$ units are called