I have a question on Java, why the answer is A not D?

Question

class X {
int x1, x2, x3;
}
class Y extends X {
int y1;
Y() {
x1 = 1;x2 = 2;y1 = 10;
}}
class Z extends Y {
int z1;
Z() {
x1 = 3;y1 = 20;z1 = 100;
}}
public class Test3 {
public static void main(String[] args) {
Z obj = new Z();
System.out.println(obj.x3 + ", " + obj.y1 + ", " + obj.z1); } }
Which constructor initlalizes the variable x3?
A) only the default constructor of class X
B) only the no-argument constructor of class Y
C) only the no-argument constructor of class Z
D) only the default constructor of object class

(The Ans is A)

Answer 1

X is the most extended superclass what contain x3.