What are the rules to implement an interface? Considering the following declaration for interface Convertable, which of the following code segments will compile?

public interface Convertable {
    int convertToInt();
    char convertToChar();
}

a)

class Digit implements Convertable {
    public char convertToChar() {
        return 0;
    }
    public int convertToInt() {
        return 0;
    }
}

b)

 abstract class Digit implements Convertable {
     int convertToInt();
     char convertToChar();
 }

c)

abstract class Digit implements Convertable {
    public int convertToInt() {
        return 0;
    }
}

d)

abstract class Digit implements Convertable {
    public int convertToInt() {
        return 0;
    }
    char convertToChar();
}

e)

  class Digit implements Convertable {
    int convertToInt() {
        return 0;
        return 0;
    }
}

f)

interface Roundable extends Convertable {
    int roundUp();
}