4.下列谓词公式中x即是约束变元也是自由变元的是
A (∀x) (A(x)→B(x))
B (∀x)( P(x)→(∃y)R(x,y))
C (∀x) (∀y) A(x,y) ∧(∃y)B(x,y)
D (∃y)( P(x)∧Q(x,y))