An Acid HB⁺ Has a K _a of 1

An acid HB⁺ has a K _a of 1.0×10 ^{- 9}. What is the p K _b of the base B?

A) 9.0
B) 7.0
C) 5.0
D) 1.0×10 ^{- 5}
E) 1.0×10 ^{- 9}