to figure out how much the vaccine is worth to determine how the vaccine will affect people to understand how that disease affects the human body to find out how many people are affected by the disease
I gave my students this multiple choice question: A vaccine can prevent a serious disease. When a new vaccine is developed, it is given to a small group of people. Why would it be important to test each vaccine on a small group of people before everyone can have one?

Only one answer is correct among these 4 choices:
- to figure out how much the vaccine is worth
- to determine how the vaccine will affect people
- to understand how that disease affects the human body
- to find out how many people are affected by the disease

Could you tell me which one is correct?

to determine how the vaccine will affect people