You are presented with the quiz "Passive immunity lasts when the antibodies survive in something containing what?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "passive immunity lasts only as long as the antibodies survive in body fluids" and "factors in Body Fluid Health Potassium and sodium are crucial to the body's fluid balance". 

So, what's the best answer to the question?


 

     potassium      