Available choices:
[A]. strong and healthy;
[B]. expelled;
[C]. infected;
[D]. decreases;
[E]. working great;
[F]. broken;
[G]. dangerous;
[H]. easygoing;
Q: You are presented with the quiz "Immunodeficiency occurs when the immune system is what?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "immunodeficiency occurs when the immune system is not working properly" and "merchandise didn t work, or was broken". So, what's the best answer to the question?
A:
[F].