Volunteer: a person who freely offers to take part in an enterprise or undertake a task., T$ n: H3 x( O! n, x4 ]* }% T
synonyms: subject, participant, case, patient; informalguinea pig , H# T( g# O+ l0 F% _4 M! Y. C% {"each volunteer was tested three times"1 g6 ?; w! O: p