在下面的运算符重载函数的原型中，错误的是( )。

A．Volume operator-(double,doubl；

B．double Volume::operator-(doubl；

C．Volume Volume::operator-(Volum；

D．Volume operator-(Volume,Volum；

Do you think maths is         than English?

A．difficult

B．as difficult

C．more difficult

D．most difficult