Which metal is used in radiation shields?

What do you understand by $(a)$ inert pair effect,$(b)$ allotropy,and $(c)$ catenation?

Give reasons: Diamond is used as an abrasive.

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 $GeX_4$.

Of the following acids, the one which has the capability to form complex compounds and also possesses both oxidizing and reducing properties is:

Identify the incorrect order against the stated property.