$(\exists x)(P(a, x) \rightarrow(\forall y) Q(x, b, y))$ 的前束范式为 $\qquad$ .
A. $(\exists x)(\forall y)(\neg P(a, x) \vee Q(x, b, y))$
B. $\neg(\forall x)(\exists y)(P(a, x) \wedge \neg Q(x, b, y))$
C. $(\exists x)(\forall y)(\neg P(a, x) \wedge Q(x, b, y))$
D. $(\exists x)(\neg P(a, x) \vee(\forall y) Q(x, b, y))$