单选题 已知类Myst有一个只需要一个double型参数的构造函数，且将运算符“-”重载为友元函数。要使语句序列
Myst x3.2), y(5.5), z(0.0);
z=8.9-y;
y=x-6.3;
能够正常运行，运算符函数operator-应该在类中声明为，
A) friend Myst operator-(Myst&,Myst&); B) friend Myst operator-(Myst,Myst);
C) friend Myst operator-(Myst,Myst&); D) friend Myst operator-(Myst&,Myst);