In the following observation.

Acid	${\mathrm{K}}_{{\mathrm{a}}_1}$	${\mathbf{K}}_{{\mathrm{a}}_2}$	(X)
Oxalic acid	$5.9\times {10}^{-2}$	$6.4\times {10}^{-5}$
Ascorbic acid	$7.4\times {10}^{-4}$	$1.6\times {10}^{-12}$
Sulphurous acid	$1.7\times {10}^{-2}$	$6.4\times {10}^{-8}$
(Y)	Very large	$1.2\times {10}^{-2}$
Carbonic acid	$4.3\times {10}^{-7}$	(Z)
Citric acid	$7.4\times {10}^{-4}$	$1.7\times {10}^{-5}$	$4.0\times {10}^{-7}$
Phosphoric acid	$7.5\times {10}^{-3}$	$6.2\times {10}^{-8}$	$4.2\times {10}^{-13}$

X,Y and Z respectively are

• A

  $\mathrm{K}={\mathrm{K}}_{{\mathrm{a}}_1}\times {\mathrm{K}}_{{\mathrm{a}}_2},\text{ phosphorous acid, }6.5\times {10}^{-11}$

• B

  ${\mathrm{K}}_{{\mathrm{a}}_3},\text{ phosphorous acid, }6.5\times {10}^{-11}$

• C

  $\mathrm{K}={\mathrm{K}}_{{\mathrm{a}}_1}\times {\mathrm{K}}_{{\mathrm{a}}_2},\text{ sulphuric acid, }5.6\times {10}^{-11}$

• D

  ${\mathrm{K}}_{{\mathrm{a}}_3},\text{ sulphuric acid, }5.6\times {10}^{-11}$