Q.Which statement from s1 to s6 (see below) cause compilation error? Assume the statement that causes compilation is deleted, what is the output when the program is executed?

class X{

void f1() { System.out.println("XXX"); }

}

class Y extends X{

void f1() { System.out.println("YYY"); }

}

class Z extends X{

void f1() { System.out.println("ZZZ"); }

}

class Test{

static void g(X a) { a.f1(); }

public static void main(String args[]){

Y y = new Y();

g(y); Object obj = new Y(); obj.f1();

X x = new Z(); x.f1();

// s1 // s2 // s3 // s4 // s5 // s6

}