She treated the boy very well _____he were her own child. [ ]

A. as though      

B. unless          

C. even though            

D. as long as

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 x(3.2),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);