定义分数类 Rational对象r1,r2,r3,成员函数声明形式与调用形式匹配的为() A. Rational Rationa:: add( const Rational& ); r3 = r1.add(r2); B. Rational Rationa::sub( const Rational& ); r3.sub(r1, r2); C. void Rationa::mul( const Rational& , const Rational& ); r3.mul(r1, r2); D. bool Rationa::div( const Rational& , const Rational&); r3= mul(r1, r2);