The element whose salts cannot be detected by flame test is

$A$ metal $(A)$ on heating in nitrogen gas gives compound $(B)$. $(B)$ on treatment with $H_{2}O$ gives a colourless gas which when passed through $CuSO_{4}$ solution gives a dark blue-violet coloured solution. $(A)$ and $(B)$ respectively,are

Which is an incorrect statement?

The blue color of a solution of sodium in liquid ammonia is due to the presence of.....

To which group do the following elements belong? Which of them has the highest metallic character?
$K, Na, Li, Rb$

Sodium gives blue colour with $NH_3$ solution,this blue colour is due to