Which of the following function headers best fits a type conversion operator to convert an object of the Fraction class to type double, so that one can use implicit type conversions such as

double root $=$ sqrt Fraction $f(1,2)$;

double operator double$($Fraction f$)$ const

double Fraction: operator$()$ const

double convert$($Fraction f$)$ const

Fraction: :operator double$()$ const