plz explain

[Naveed Ali]

Given the following class definition.
class A{
protected int i;
A(int i) {
this.i = i;
}
}
Which of the following would be a valid inner classes for this class? Select all valid answers.
a) class B{}
b) class B extends A {}
c) class B{
B() {
System.out.println("i = " + i);
}
}
d) class B{
class A {}
}
e) class A{}
Can any body explain plz why the valid answers are only (a) & (c) and other are not.
Thnx.