Fact 1: Passive immunity lasts only as long as the antibodies survive in body fluids.

Fact 2: Booster shots are needed periodically to build up the passive immunity again.

Given the two facts above, answer the question "What are needed after antibodies won't survive in body fluids?" with the following options: 
- booster shots
 - Veins and arteries.
 - chicken pox
 - Energy.
 - gun shots
 - kinetic energy
 - diseased tissue
 - turn to a doctor


 

 booster shots        