利用谓词的约束变元的换名规则和自由变元的代入规则，可将公式改写成______．

A.(x)(P(y)→Q(x，y))∧R(z，s)

B.(z)(P(z)→Q(z，s))∧R(x，s)

C.(x)(P(s)→Q(x，s))∧R(x，s)

D.(z)(P(s)→Q(z，s))∧R(z，s)