Identify a species which is not a Bronsted acid but a Lewis acid.

In the reaction where $HNO_3$ acts as a base,which of the following acts as the strongest acid?

Lemon gives a sour taste because of

Given:
$(i) \ HCN_{(aq)} + H_2O_{(l)} \rightleftharpoons H_3O^{+}_{(aq)} + CN^{-}_{(aq)}$
$K_a = 6.2 \times 10^{-10}$
$(ii) \ CN^{-}_{(aq)} + H_2O_{(l)} \rightleftharpoons HCN_{(aq)} + OH^{-}_{(aq)}$
$K_b = 1.6 \times 10^{-5}$
These equilibria show the following order of the relative base strength:

Phosphoric acid ionizes in three steps with their ionization constant values $K_{a_1}, K_{a_2}$ and $K_{a_3}$ respectively,while $K$ is the overall ionization constant. Which of the following statements are true?
$A.$ $\log K = \log K_{a_1} + \log K_{a_2} + \log K_{a_3}$
$B.$ $H_3PO_4$ is a stronger acid than $H_2PO_4^{-}$ and $HPO_4^{2-}$
$C.$ $K_{a_1} > K_{a_2} > K_{a_3}$
$D.$ $K_{a_1} = \frac{K_{a_3} + K_{a_2}}{2}$
Choose the correct answer from the options given below

Considering the following reactions,which pair of reactions shows that water has basic properties?
$(i) \ CO_2 + H_2O \rightleftharpoons H_2CO_3$
$(ii) \ NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-$
$(iii) \ HCl + H_2O \rightleftharpoons Cl^- + H_3O^+$