K_a for HF Is 7

K_a for HF is 7.0 × 10^{- 4}. K_b for the fluoride ion is _ _.

A) 7.0 × 10^{- 4}
B) 2.0 × 10^{- 8}
C) 1.4 × 10^{- 11}
D) 1.4 × 10³
E) 7.0 × 10^{- 18}