What Is the K_a for Nitrous Acid, Given That the PK_b

What is the K_a for nitrous acid, given that the pK_b for the nitrite ion is 10.61?

A) 1.38 $\times$ 10⁴
B) 4.07 $\times$ 10^-4
C) 1.90 $\times$ 10^-5
D) 5.60 $\times$ 10^-6
E) 4.30 $\times$ 10^-7