Objects eligible for garbage collection

This question was taken from Kathy Sierra SCJP 1.6. How many objects are eligible for garbage collections?

According to Kathy Sierra's answer, it is C. That means two objects are eligible for garbage collection. I have given the explanation of the answer. But why is c3 not eligible for garbage collection (GC)?

class CardBoard {
    Short story = 200;
    CardBoard go(CardBoard cb) {
    cb = null;
    return cb;
}

public static void main(String[] args) {
    CardBoard c1 = new CardBoard();
    CardBoard c2 = new CardBoard();
    CardBoard c3 = c1.go(c2);
    c1 = null;
    // Do stuff
} }

When // Do stuff is reached, how many objects are eligible for GC?

• A: 0
• B: 1
• C: 2
• D: Compilation fails
• E: It is not possible to know
• F: An exception is thrown at runtime

Answer:

• C is correct. Only one CardBoard object (c1) is eligible, but it has an associated Short wrapper object that is also eligible.
• A, B, D, E, and F are incorrect based on the above. (Objective 7.4)