Assertion (A): `Na_(2)S_(2)O_(3)` is a salt of unstable acid.
Reason (R) : `H_(2)S_(2)O_(3)` is a polyprotic acid.
A. If both (A) and (R) are correc, and (R) is the correct explanation of (A).
B. If both (A) and (R) are correct but (R) is not the correct explanation of (A).
C. If (A) is correct, but (R) is incorrect.
D. If (A) is incorrect, but (R) is correct.