You should add the static keyword in the place of ? in Line ________ in the following code:

```
public class Test {
private int age;

public ? int square(int n) {
return n * n;
}

public ? int getAge() {
}
}
```
a. in line 4
b. in line 8
c. in both line 4 and line 8
d. none

a. in line 4
The square method should be static because it does not reference any instance data or invoke any instance method.