Which line causes a compilation error?

Given:

public class ScopeTest1 {
public static void main(String[] args) {
doStuff(); // line x1
int x1 = x2; // line x2
int x2 = j; // line x3
}
static void doStuff() {
System.out.println(j); // line x4
}
static int j;
}

Which line causes a compilation error?

A.
line x1

B.
line x2

C.
line x3

D.
line x4

Explanation:

The variable x2 is used before it has been declared.