A paper is dipped in a dil. $$\text{H}_2\text{SO}_4$$ solution of  'X' upon treatment with $$\text{SO}_2$$ gas turns into green.The compound  'X' is :

A

KI-starch

B

$$\text{KMnO}_4$$

C

$$\text{Pb(CH}_3\text{COO)}_2$$

D

$$\text{K}_2\text{Cr}_2\text{O}_7$$