Copper sulphate solution reacts with $KCN$ to give

Given the following reactions:
$Ag^{+} + NH_3 \rightleftharpoons [Ag(NH_3)]^+ ; K_1 = 1.6 \times 10^3$
$[Ag(NH_3)]^+ + NH_3 \rightleftharpoons [Ag(NH_3)_2]^+ ; K_2 = 6.8 \times 10^3$
What is the formation constant for $[Ag(NH_3)_2]^+$?

If an excess of $NH_4OH$ is added to a $CuSO_4$ solution,it forms a deep blue coloured complex which is:

Which of the following represents the correct increasing order of the spectrochemical series (field strength) for some common ligands?

Complex $A$ has a composition of $H_{12}O_{6}Cl_{3}Cr$. If the complex on treatment with conc. $H_{2}SO_{4}$ loses $13.5\%$ of its original mass,the correct molecular formula of $A$ is :
[Given : atomic mass of $Cr = 52 \ amu$ and $Cl = 35.5 \ amu$]

Select the most stable complex.