In which of the following is a metal-to-metal bond present?

Match column-$I$ with column-$II$ :

column-$I$	column-$II$
$a$. $Ra$	$p$. Aeroplane industry
$b$. $K$	$q$. Down cell
$c$. $Li$	$r$. Photo electric cell
$d$. $Na$	$s$. Radioactive

Copper becomes green when exposed to moist air for a long period. This is due to

Assertion : $Al(OH)_3$ is insoluble in $NH_4OH$ but soluble in $NaOH$.
Reason : $NaOH$ is a strong alkali.

Which of the given orders is not correct?

Which of the following orders are correct against the stated property?
$I$) $NaO_2 < KO_2 < RbO_2 < CsO_2$ - stability
$II$) $Mg(OH)_2 < Ca(OH)_2 < Sr(OH)_2$ - basic strength
$III$) $MgCO_3 < CaCO_3 < SrCO_3$ - thermal stability