You are presented with the quiz "What are needed after antibodies won't survive in body fluids?" 

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 "booster shots are needed periodically to build up the passive immunity again". 

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


 

 booster shots          