Volunteer: a person who freely offers to take part in an enterprise or undertake a task.7 Q4 v5 ?$ S; u! K: {7 b
synonyms: subject, participant, case, patient; informalguinea pig! u9 f9 q2 t/ V6 O3 I* c7 o- K
"each volunteer was tested three times" ! M+ @# T- E9 K$ t& d) A