given
1. class Dog{}
2. class Beagle extends Dog{}
4. class Kennel{
5. public static void main(String[] arfs)
6. Beagle b1 = new Beagle();
7. Beagle b2 = new Dog();
8. Dog dog1 = new Dog();
9. Dog dog2 = b1;
10. Dog dog3 = new Beagle();
}
}

which line will have a compilation error?